Earth-Science Reviews 57 Ž2002. 37–74
www.elsevier.comrlocaterearscirev
The two-stage concept of landform and landscape development
involving etching: origin, development and implications of
an idea
C.R. Twidale )
Department of Geology and Geophysics, UniÕersity of Adelaide, G.P.O. Box 498, Adelaide SA 5005, Australia
Received 5 January 2001; accepted 28 March 2001
Abstract
Two-stage development of landforms has been appreciated for more than two centuries with respect to minor features,
and major forms and landscapes have been viewed in similar terms for almost a hundred years. Early workers understood the
significance of fractures as passages for water and, hence, as avenues of weathering, the tendency for weathering to produce
rounded forms, the progression of weathering from the surface downwards, weathering contrasts between wet and dry sites,
contrasted erosive susceptibility of weathered and unweathered rock, and reinforcement effects. Forms of deep and shallow
derivation can be differentiated.
By whatever name it is known—etch, double planation, subcutaneous, or two-stage—the concept is one of the most
fruitful developed in the last century, for it bears not only on the origin of a wide range of landforms, but also on the crucial
role of water and weathering, the age of landforms and landscapes, palaeogeographic reconstructions, climatic geomorphology and theories of landscape development. q 2002 Elsevier Science B.V. All rights reserved.
Keywords: etching; two-stage development; double planation; regolith; weathering front; azonality
1. Introduction
Over the past 50 years, it has increasingly been
recognised that substantial components of the world’s
landscapes were shaped not at the Earth’s surface,
but at the base of the regolith. Such forms are the
result of differential weathering produced by shallow
groundwaters held in the weathered mantle and reacting with the local country rock. Exploitation of
weaknesses in the bedrock produced a morphologi-
)
Tel.: q61-8-8303-5392; fax: q61-8-8303-4347.
E-mail address: [email protected] ŽC.R. Twidale..
cally differentiated surface or alternatively, a plane
surface, in either instance followed by stripping of
the regolith and exposure of the bedrock form. Many
familiar landforms, from boulders to bornhardts, from
basins to gutters and flared slopes, and from platforms to plains, originated in this way.
If the geomorphological value of an hypothesis is
the number of landforms and landscapes it explains,
then the two-stage concept is surely one of the most
significant, if not the most important, to have
emerged over the past 50 years or so. How has it
changed our perception of landscape, and when and
by whom was this mechanism, which is referred to
pro tempore as two-stage, envisaged? The recogni-
0012-8252r02r$ - see front matter q 2002 Elsevier Science B.V. All rights reserved.
PII: S 0 0 1 2 - 8 2 5 2 Ž 0 1 . 0 0 0 5 9 - 9
38
C.R. Twidaler Earth-Science ReÕiews 57 (2002) 37–74
tion of the mechanism and its naming, its derivation
from, and application to, landforms at various scales
and in various bedrocks, and its implications for
general theories of landscape development are considered, as are some of the difficulties inherent in the
concept.
This review and critique is perforce based primarily on a consideration of the English-language literature. The writer’s restricted library resources and his
own linguistic limitations precluded the possibility of
browsing over a wide range of old reports and
narratives and journals of scientific travellers. This is
a weakness, for many observations and comments
critical to the development of the two-stage concept
were recorded incidentally, and are not captured in
abstracts or keyword indices. The two-stage concept
may have been realised earlier and elsewhere than is
suggested here.
2. Corestones and other minor forms in granite
Though other bedrock types were not ignored—as
demonstrated by Sain Fond’s Ž1784. observations of
basaltic corestones in Scotland and various accounts
of orgues geologiques
in the Chalk lands of western
´
Europe—granitic terrains received considerable at-
tention from early geologists, possibly because of
associated mineral deposits, and the main thrust for
various early ideas related to two-stage development
arose from observations in such areas. One of the
earliest suggestions of two-stage development is due
to Hassenfratz Ž1791.. Late in the 18th Century and
travelling in the southern Massif Central near Aumont, between St. Flour and Montpellier, he observed rounded masses of fresh granite protruding
from a bank of weathered rock ŽFig. 1.. Some were
only just visible, others partly exposed, and yet
others detached. Hassenfratz deduced that the freestanding blocks and boulders had once been covered,
and suggested that the assemblage were members of
a sequential series:
entre un
. . . on apperçoit tous les intermediaires
´
bloc de granit dur contenu & enchasse´ dans la
masse totale du granit friable & un bloc entiere`
ment degage.
´ ´ ŽHassenfratz, 1791, p. 101.. wAll
stages can be observed between a block of granite
totally contained within the mass of altered rock
and one wholly detached.x
Hassenfratz’s comments were noted by Hutton
Ž1795, II, p. 174., who recorded that such boulders
originated Aby the decay of the rock around themB
Fig. 1. It was at this site, or one near and like it, at Chazeirollettes, in the southern Massif Central, that Hassenfratz in 1791 saw granitic
corestones in various stages of exposure.
C.R. Twidaler Earth-Science ReÕiews 57 (2002) 37–74
rather than being expressions of structure Žvaried
composition of the rock. as suggested by some Že.g.,
Jameson, 1820, p. 414.. The same interpretation was
placed on granite boulders by several, though not all,
later workers in Britain ŽTwidale, 1978a..
These early investigators were astute observers
and were responsible for great advances. The significance of fractures as avenues of weathering, and the
role of water in altering the bedrock were understood, as was the tendency for cubic and quadrangular blocks to be converted to rounded corestones Žor
core-stones—Fig. 2., which, at various times, were
known also as kernels and Ahearts of the blocksB
ŽJones, 1859, p. 307., and later, core-boulders
ŽScrivenor, 1931., and boulders of disintegration
ŽLarsen, 1948, p. 114 et seq... Thus, MacCulloch
Ž1814, p. 76. observed that granite blocks Anow
rendered spherical by decomposition, have been
quadrangular massesB wor as it was also expressed Žp.
74. ANature mutat quadrata rotundisB x and de la
Beche Ž1839, p. 450. pointed out that:
The granites are separated by divisional
planes . . . into cuboidal or prismatic bodies, and
the decomposition on the faces of these bodies,
when the blocks are detached, and the superior
facility for disintegration afforded by the corners
39
would appear sufficient to produce the rounded
character we often observe them to possess.
At that time, de la Beche apparently considered
that the shaping of the boulders took place after
exposure, but later, de la Beche Ž1853. published a
sketch depicting rounded corestones in situ within a
weathered mantle. Others made similar observations
and drew similar conclusions.
It was also realised that weathering advances from
the surface downwards and, thus, that initial stages
of change are to be observed at the base of the
mantle and that the higher in the profile, the more
advanced the process:
a change may always be observed to have taken
place from the surface downwards to a more or
less considerable depth in the stone. Sometimes
even the whole mass of rock will appear to have
undergone this gangrenous process at once, and to
have become a bed of clay and gravel. ŽMacCulloch, 1814, p. 72..
Working on the other side of the world, Logan
reached similar conclusions, commenting that:
The blocks protruding from the hills or ranged
along the shores of Pulo Ubin wproperly, Pulau
Fig. 2. Core-boulders, or corestones, in grus on Karimun Island, western Indonesia, with lamination at the margins of the rounded rock
masses.
40
C.R. Twidaler Earth-Science ReÕiews 57 (2002) 37–74
Ubin in the Strait of Johor, and off the north coast
of Singaporex are more solid and less decomposable masses and nuclei, of which the forms, and
the directions of the sides and axes, have, in
almost every instance, been determined by structural planes, and which remain after the surrounding rocks have disintegrated and been washed
away. ŽLogan, 1849, p. 40..
Some early geologists, including Ansted Ž1859.
working in southern China, Agassiz Ž1865. in the
Rio de Janeiro area, AW.T.B Ž1896. on the Malay
Peninsula and Romanes Ž1912. in Costa Rica, considered that granite boulders had been rounded during transport either by rivers or glaciers, but evidence demonstrating that they were in situ was
recognised ŽFig. 3.. For example, travelling in the
uplands near Canton, in southern China, Kingsmill
observed corestones set in a matrix of grus and noted
that:
The original quartz veins of the granite, broken
into small fragments by the forces which have
operated on the surrounding rock, still traverse the
disintegrated mass in all directions. ŽKingsmill,
1862, p. 2..
Contrasts in degree of weathering at moist and
dry sites were recognised 150 years ago ŽLogan,
1851, p. 326.. Thus, once corestones are exposed as
boulders, they are relatively dry and stable Žsee also
Ruxton and Berry, 1957.. Plinths ŽFig. 4. are due to
the protection against wetting afforded by what have
become perched blocks and boulders.
Core-boulders are discrete parts of the weathering
front ŽMabbutt, 1961a., the abrupt transition between
regolith and bedrock, examples of which were noted
in Malaya and elsewhere, as were spatial variations
in degree of weathering in granitic masses ŽPumpelly,
1879; and later, e.g., Scrivenor, 1931; Ingham and
Bradford, 1960.. Scrivenor Ž1931, p. 137., for example, noted that though in Malaysia the regolith in
places grades into fresh rock, elsewhere there is an
abrupt transition, so that the granite forms what he
called a Ahard platformB beneath the weathered mantle. Several early investigators noted lamination at
the margins of granitic corestones ŽFig. 2.. Working
on Pulau Ubin, Logan, for instance, refers to plates
spalling from the granite surface, and consisting of
laminae a quarter to a fifth of an inch thick ŽLogan,
1849, p. 12.. Such textures represent an early stage
of weathering and are typical of the weathering front
Fig. 3. Corestones and boulders near Pine Creek, Northern Territory. The aplitic vein shows that the section is in situ.
C.R. Twidaler Earth-Science ReÕiews 57 (2002) 37–74
41
Fig. 4. Plinth and perched block developed in a Miocene rhyolitic tuff at the Giants of the Mimbres, southwestern New Mexico, USA. ŽJ.E.
Mueller..
Že.g., Larsen, 1948, p. 115; Hutton et al., 1977;
Twidale, 1986..
That the implied differences in rates of weathering induced feedback or reinforcement mechanisms
was also appreciated. Thus, describing the results of
wave attack on the shores of Pulau Ubin washing
away the weathered granite, Logan Ž1851, p. 326.
states that this left:
the more resistant masses to emerge from the soil
and stand out above the influence of decomposition. wfollowed, in a footnote by:x When an exposed rock is attacked, the decomposing portion
is washed or falls off, and the decomposition is
arrested for the time. Under-ground decomposition tends to spread unchecked on all sides.
On Pulau Ubin, Logan Ž1849, 1851. observed that
many granite boulders are scored by grooves Ž Rillen,
Granitrillen, Silikatrillen. and noted Ž1849, p. 6. that
some, Aif not mostB, continue beneath the weathered
granite in situ, which extended to considerable depths
beneath the slopes around the blocks and boulders.
This observation has not been replicated for steeply
inclined surfaces, but on more gentle slopes, gutters
have been observed to extend below soil level at
several sites ŽTwidale and Bourne, 1975a.. Logan
Ž1851, p. 328. considered that the grooves corresponded with zones of weakness in the rock and,
though such a structural interpretation does not find
support in the field, he put the argument for subsurface initiation thus:
If . . . we conceive the external layer of the island,
when it first became exposed to decomposition, to
have resembled in character the zone that has
been laid open for our inspection . . . it is easy to
comprehend how the wasting away of the more
decomposable parts might at last leave exposed
masses, including bands of the less stubborn material already partially softened or disintegrated
under ground, and that the action of the atmosphere and rain-torrents would gradually excavate
the more yielding portions, until the solid remnants exhibited their present shapes.
The rapidity with which rocks were decayed in
the humid tropics, and the capriciousness of nature,
also impressed early investigators. Scrivenor Ž1931,
pp. 136–137., for instance, noted a small boulder of
granite split by blasting and exposed in a road
cutting in the uplands NNE of Kuala Lumpur. Fresh
42
C.R. Twidaler Earth-Science ReÕiews 57 (2002) 37–74
C.R. Twidaler Earth-Science ReÕiews 57 (2002) 37–74
when first exposed, it was rotted through within a
decade Žcf. Caillere
` and Henin, 1950.. Elsewhere,
however, fresh and rotted rocks, stand side by side.
3. Two-stage development of minor forms in other
lithologies
Two-stage boulders are also well and widely developed in other lithological environments. Well developed and moderately spaced Žideally 1–3 m or so.
orthogonal fracture systems, low permeability and a
susceptibility to chemical weathering, are conducive
to their formation. They occur in norite in the Mann
Ranges of central Australia and elsewhere Že.g., Hutton et al., 1977., and in many other plutonic rocks.
Sain Fond had in 1784 noted basaltic corestones with
marginal lamination in Scotland and similar basaltic
core-boulders are well exposed in many parts of the
Drakensberg of southern Africa ŽFig. 5a.. Corestones
have been reported in sandstone in the southern
Flinders Ranges of South Australia and in the Appalachians of Tennessee, and in gritstone in the
English Pennines ŽPalmer and Radley, 1961; Linton,
1964.. Many other minor sandstone forms—flared
slopes, platforms, basins, gutters—are also of twostage type Žsee Twidale, 1962, 1980; Young and
Young, 1992; Fig. 5b..
Though the subsurface initiation of granitic forms
attracted early attention, carbonates are more readily
soluble than most other rocks and two-stage forms
are well represented in karst terrains. Congeners of
corestones, for example, are well developed in limestone. Most are irregular and blocky rather than
spheroidal in shape and after exposure are known as
Karrenblocke
and Karrensteine. Moreover, mor¨
phology varies in detail according to whether a
surface is exposed or covered, Rundkarren, for
example, developing under a soil cover, but Rinnenkarren on bare surfaces. Indeed, karst assemblages have been differentiated on the basis of devel-
43
opment beneath a cover or after exposure Že.g.,
Sweeting, 1972, pp. 252 et seq., especially p. 298.,
for example, bare karst Ž nackter karst., karst couÕert ŽCorbel, 1947., covered karst Ž bedeckter karst
or limestone surfaces overlain by allochthonous materials, and cryptokarst ŽNicod, 1976: cited in Jennings, 1985, p. 80. or limestone covered by a soil or
regolith derived from weathering of the country rock.
According to Corbel Ž1947., covered karst differs
from bare Ž nackter . karst in several respects, but
principally in the prime direction of solution: vertical
on bare surfaces, lateral on covered. Such distinctions, however, are not in accord with all the field
evidence. The earliest covered karst forms described
in the literature were the pipes of the Chalk lands of
western Europe and variously known as swallow
holes, gulls, puits naturels, orgues geologiques,
and
´
geologische Orgeln ŽCuvier and Brogniart, 1822;
Buckland, 1839; Lyell, 1840; Prestwich, 1855; Van
den Broek, 1881.. They are clearly due to vertical
solution and infilling, as are the prongs found in
crystalline limestone, typically beneath a terra rossa
soil cover ŽFig. 5c.. Ingham and Bradford Ž1960, p.
30., for example, report prongs and pinnacles, many
of them with flared or concave sidewalls, developed
in limestone beneath 30 m of regolith in the Ipoh
district of West Malaysia. Some of the limestone
surface, formerly the weathering front, carries a coating of siderite, or ferruginous carbonate comparable
to the ferruginous weathering front concentrations
found in granitic terrains Žsee Twidale, 1986..
4. Major forms: bornhardts and inselberg landscapes
4.1. Early reports
By the beginning of the 20th Century, the subsurface origin of several minor forms had been noted
and, in some instances, as with corestones, widely
Fig. 5. Ža. Corestones in basalt, with peripheral lamination, southern Drakensberg, Eastern Cape Province, South Africa. Žb. Gutters related
to decantation flows on a recently exposed gentle sandstone slope, in the southern Drakensberg. Žc. Limestone prongs exposed beneath terra
rossa soil in Galong Quarry, central New South Wales. Note also the rough surface, the equivalent of pitting in granitic rocks, caused by the
etching of calcite crystals, possibly in the zone of fluctuating water table.
44
C.R. Twidaler Earth-Science ReÕiews 57 (2002) 37–74
accepted. Thus, the intellectual climate had been
prepared for the consideration of the two-stage
mechanism in the context of larger landforms and of
landscapes.
Explorers of the Australian interior had encountered astonishingly flat plains interrupted by scattered, isolated, steep-sided hills they likened to rocky
islands and which they called island mounts Že.g.,
Eyre, 1845, I, p. 203.. Such features were encountered by German travellers in southern Africa and
they used the same simile:
. . . there rose above the wide plain innumerable,
peculiarly shaped hills, resembling islands, steep
and rocky and several hundred, yes in certain
cases more than 500 meters high . . . ŽBornhardt,
1900, pp. 127–128: translated by Willis, 1934, p.
123..
They referred to them as Inselberge; and it is this
name that is now universally accepted in the scientific literature.
Domical granitic hills, now widely known as
bornhardts, after the German explorer of that name
ŽWillis, 1934., were described by several explorers
and scientific travellers during the 19th Century.
Several explanations were offered for them and for
the landscapes in which they stood. Bornhardt Ž1900.
briefly considered the possibility of their being literal
island mounts, but soon came to the view that repeated phases of fluvial planation were involved.
Passarge Ž1895, 1904. interpreted the inselberg landscapes of Adamaua ŽWest Africa. as of four types,
some due to wind abrasion, some to a combination
of aeolian and fluvial action. Holmes Ž1918, p. 93.
advocated scarp recession under the influence of
water and wind, in some instances, resulting in the
exposure of intrusive stocks ŽHolmes and Wray,
1912.. The abrupt transition from cohesive to rotten
rock Žwhich was known to extend several tens of
metres below the surface. was used to explain the
morros of the Rio de Janeiro area ŽBranner, 1896.;
and, indeed, the answer to the problem of the origin
of the residuals lay not at, but beneath, the surface.
4.2. Pumpelly, Falconer, and subsurface initiation
Pumpelly was a widely travelled geologist and
explorer. His observations and reading led him to the
conclusion that weathering in places extended to
great depths ŽPumpelly, 1879.. He also had the wit
to consider what would be found if the mantle of
weathered rock were removed, and wrote:
As different rocks are affected in very different
degrees . . . the plane marking the boundary between disintegrated rock and still hard rock wthe
weathering frontx must be an exceedingly irregular one.
If we could imagine the loose altered rock removed where this process has been active in
depth, the surface exposed would present a remarkable topography . . . ŽPumpelly, 1879, pp.
136–137.
He clearly contemplated two stages of development,
with weathering followed by the evacuation of the
products of disintegration and alteration, and had in
mind ŽA . . . a remarkable topography . . . B . landform
assemblages or landscapes, rather than individual
forms.
A two-stage concept was used to explain the
spectacular fields of inselbergs found in many parts
of Africa; though without reference to Pumpelly’s
speculations. Basing his synthesis on his studies of
Nigerian landscapes, Falconer Ž1911. suggested that
the landscapes are due to subsurface moisture attack
exploiting bedrock contrasts and that the salient features of inselberg landscapes are exposed weathering
fronts. He noted that within the walls of Kano, the
flat-topped dioritic hill known as Kogan Dutsi included corestones and he asserted that had erosion
continued to evacuate all weathered material, it would
have become a hill of loose boulders resting on a
smooth rounded base. ŽFalconer referred to the form
as a AkopjeB Žor ‘koppie’., Afrikaans for a small hill,
but it would today be called a nubbin or knoll, the
humid tropical development of the bornhardt; see
Twidale, 1981.. He was able to imagine future events
and envisage their results. In a masterly and succinct
outline of a two-stage theory of inselberg landscape
development, he stated:
A plane surface of granite and gneiss subjected to
long-continued weathering at base level would be
decomposed to unequal depths, mainly according
to the composition and texture of the various
C.R. Twidaler Earth-Science ReÕiews 57 (2002) 37–74
rocks. When elevation and erosion ensued, the
weathered crust would be removed, and an irregular surface would be produced from which the
more resistant rocks would project. Those rocks
which had offered the greatest resistance to chemical weathering beneath the surface would upon
exposure naturally assume that configuration of
surface which afforded the least scope for the
activity of the agents of denudation. In this way
would arise the characteristic domes and turtlebacks . . . ŽFalconer, 1911, p. 246..
Though bornhardts develop for various reasons, a
wide range of evidence and argument suggests that
the two-stage theory accounts for most. Compositional differences of various origins have been exploited, in some instances Že.g., Holmes and Wray,
1912; du Toit, 1937, e.g., p. 61; Herrmann, 1954;
Thorp, 1969; Selby, 1977; Hagedorn, 1980, p. 833.,
but contrasts in fracture density have most frequently
led to differential weathering. Le Conte Ž1873, p.
324. and Mennell Ž1904, p. 74. had earlier pointed to
this factor in explanation for domes being upstanding, but had not envisaged exploitation prior to
exposure. That was Falconer’s great contribution.
Evidence and argument supportive of the twostage hypothesis are many and varied ŽTwidale,
1982a, pp. 139–149, 1982b; Vidal Romani and
Twidale, 1998.. For instance, the relationship between the residuals and old planation surfaces is
45
suggestive, for the former commonly stand lower in
the landscape than the latter, and King Ž1949, pp. 84
and 85, referring to the work of Obst, 1923. cites
this relationship between planation surface and residual as characteristic of bornhardts Žsee also Handley,
1952; Lister, 1976, 1987; Whitlow, 1978–1979..
That bornhardts occur in multicyclic landscapes can
be construed as indicating first, that there has been
time for differential subsurface weathering, and second, that there has been deep erosion of the land
mass to expose compressive zones of antiformal
structures, the convex-upward sets of sheet structures
which are typical of bornhardts Žsee, e.g., Dale,
1923; Lamego, 1938; King, 1949; Birot, 1958;
Twidale et al., 1996.. Perhaps the most telling
evidence, however, is the occurrence of incipient
bornhardts, already shaped in the subsurface ŽFig. 6.,
and exposed in quarries and road cuttings ŽBoyé
and Fritsch, 1973; Twidale, 1982a, pp. 142–144,
1982b..
Residuals already shaped at the weathering front
have been detected also in limestone terrains. In the
Kuala Lumpur district of West Malaysia, for example, geophysical surveys to investigate foundation
conditions for the Sepang international airport disclosed steep-sided limestone projections up to 50 m
high and reaching to within 20 m of the land surface
in one instance, but more commonly 40 m ŽFig. 7..
They are bordered by deep bedrock depressions occupied by weathered limestone, and the whole se-
Fig. 6. Large-radius dome exposed at Elkington Quarry near Minnipa, northwestern Eyre Peninsula, South Australia.
46
C.R. Twidaler Earth-Science ReÕiews 57 (2002) 37–74
Fig. 7. Profile showing morphology and distribution of Palaeozoic limestone bedrock, weathering mantle and alluvium at Sepang, near
Kuala Lumpur, West Malaysia, based on a seismic refraction survey. Vertical exaggeration approximately= 4. Žafter Ho, 1993..
quence is buried beneath 40–60 m of alluvium Že.g.,
Ho, 1993.. Within the city limits of Kuala Lumpur,
the proposed site of the Twin Towers had to be
moved in order to place the load centre on a limestone, rather than a deep soil, foundation.
Nascent domes or towers have not so far been
exposed in artificial excavations in sandstone or
conglomeratic strata, but many steep-sided remnants
in arenaceous and rudaceous rocks stand below
palaeosurface remnants, and other evidence and argument similar to that derived from the consideration
of granitic forms is germane. For example, the flared
flanks of the Meteora towers, located in central
Greece, and eroded in Miocene conglomerate, argue
subsurface weathering and exploitation of a massif
Žsee Twidale, 1962.. Again, the sandstone towers of
the Bungle Bungle and George Gill ranges, in northwestern and central Australia respectively, not only
stand below prominent planation surfaces, but they
display mineral concentrations ŽYoung, 1986. compatible with weathering front accumulation. They are
two-stage forms.
4.3. Jutson and the Western Australian inselberg
landscape
rocks ŽFig. 8.. Weathering and planation produced
the duricrusted Old Plateau. Following uplift and
river rejuvenation, the regolith was extensively
stripped to expose as the New Plateau the erstwhile
weathering front related to the formation of the Old.
Mesa remnants of the Old Plateau, as well as numerous granitic domes, stand on the New Plateau or high
plain in an extensive inselberg landscape. Later
workers have built on this foundation and have
detailed the extent and relationship of the surfaces
Že.g., Woolnough, 1918; Mabbutt, 1961b; Finkl and
Churchward, 1973; Mulcahy, 1973.. Also, Walther
Ž1915. realised that if the age of the lateritised
surface could be determined, the rates at which
subsequent valley incision and extension through
scarp retreat could be established. Examination of
valley deposits suggests that the Old Plateau predates
the Eocene and that the headward recession of valleys to form the New Plateau can be plotted from
Eocene in and near antecedent and inherited valleys,
through Miocene and Pliocene in the middle and
headward reaches ŽCommander, 1989; Clarke, 1994;
Waterhouse et al., 1995; Salama, 1997; Twidale and
Bourne, 1998..
4.4. Willis, Wayland and etching
Three years after the appearance of Falconer’s
seminal hypothesis, Jutson Ž1914. published an explanatory description and critique of the landscapes
of Western Australia. Inter alia, he proposed a twostage explanation for the Old and New plateaux of
the southern Yilgarn Craton, developed mainly on
granitic rocks and ‘greenstones’, or basic igneous
Thanks to the work of early investigators, the
two-stage origin of some landforms was appreciated
almost a century ago. The concept and, in particular,
the process or processes involved did not, however,
have a name. Two-stage development was clearly
implied by early workers investigating boulders and
C.R. Twidaler Earth-Science ReÕiews 57 (2002) 37–74
47
Fig. 8. Jutson’s diagrams showing the development and relationship of the Old and New plateaux, Western Australia. ŽJutson, 1914, p. 143..
corestones and the term was widely used informally.
The first formal naming of the concept, however, is
due to Willis and Wayland, working in East Africa
in the late twenties and early thirties of the last
century.
To ‘etch’ is to engrave by eating away by acids
or, more generally to corrode a surface by means of
aggressive reagents. Bearing in mind the attack of
bedrock by shallow groundwaters armed with chemicals and biota, it is an apt and evocative term for the
process involved in the development of the various
two-stage bedrock features described above. But it
was not a term that sprang readily to mind and its
history is rather involved.
48
C.R. Twidaler Earth-Science ReÕiews 57 (2002) 37–74
Both the name ‘etch’ or ‘etched’ plain, and by
implication the associated concept, are usually attributed to Wayland Žsee, e.g., Thornbury, 1954, p.
193; Thomas, 1994, p. 287., but apart from the
mechanism having been earlier recognised, Wayland
himself gives a different account of the genesis of
the name. Though he did not publish it until 1936,
Willis Ž1936, p. 124. had evidently earlier used the
term ‘etch’ in its geomorphological sense of forms
initiated at the weathering front and later exposed.
According to Wayland, writing in the Uganda Geological Survey Annual Report and Bulletin for the
year ending 31 March 1934, it was coined Athe other
day by Bailey Willis who aptly names this erosive
operation etching B ŽWayland, 1934, p. 79.. Willis
worked in the field on the Rift Valley in 1929,
suggesting that, unless Wayland applied geological
scales of time to everyday events, Athe other dayB
most likely refers to correspondence. Willis Ž1936.
may have shown his colleague a draft of what
eventually became his monograph on East African rifts and plateaux. Nevertheless, Wayland
Ž1934, pp. 77–79. clearly understood the twostage mechanism. Indeed, he claimed he had endeavoured to explain it 14 years previously ŽWayland, 1921, p. 40, para. 165., but it is fair to
comment that his statement on the point at that
time was brief and vague, rather than explicit
and unmistakeable, and is more relevant to the question of unequal activity than to etching Žsee below,
Section 5.6..
Wayland properly and generously acknowledged
the provenance of the term ‘etch’, and it is worthwhile examining Willis’ 1936 monograph on that
account alone. Willis Ž1936, pp. 118–121. first presented evidence for deep weathering in East Africa
and elsewhere, but he was also aware of the propensity of masses of resistant rock to persist and to
increase in relief through successive phases of
weathering and lowering of the adjacent plains. He
discussed etching and its consequences, without
defining or mentioning the term until p. 124, when
he offered it as an alternative to the then conventional wisdom:
The fact that East Africa has been raised is
demonstrable on the evidence of the plateau landscapes, whether they be interpreted as peneplains
or as etched plains with inselbergs, . . . ŽWillis,
1936, p. 124..
Then, however, Willis Ž1936, p. 134. categorically stated that particular exposed bedrock surfaces
Arepresent the former contact between saprolite, the
product of decay, and the fresh undecayed rockB and
on p. 135, remarked that A . . . the rock floor of the
Tanganyika Plateau may be described as the etched
surface produced by the penetration of decay to
groundwater level below an older plainB. Thus,
though not as succinct as Falconer Žwhose work was
not cited., Willis clearly interpreted some of the East
African land surfaces as of subsurface derivation,
and of two-stage type.
Willis presented his evidence and also suggested
AetchingB as an appropriate and evocative term for
the process; and indeed it is not difficult to imagine
shallow groundwaters charged with chemicals and
biota eating into the bedrock with which they are
in contact, eventually to produce, and to utilise
MacCulloch’s Ž1814. wonderful metaphor, a
AgangrenousB mass.
4.5. Linton, tors and inselbergs
The two-stage or etch hypothesis was well understood by the 1940s, not only through the writings of
the authors mentioned, but also through discussion of
the ideas of Jutson and Wayland, for example, in
some British and Australian universities and at scientific meetings Že.g., Hills, 1962.. Yet it was not
everywhere appreciated. Many texts of the early and
mid 20th Century referred to multicyclic landscape
development, but few to two-stage development of
features of regional extent, either tacitly or by name,
and if the idea were noted, it was underplayed.
Thornbury Ž1954, p. 193., for instance, cited Wayland’s etch concept, but concluded that it was of
local importance only: A . . . it is difficult to visualize
the process operating widely enough to produce
etchplains of regional extentB. Today, the concept is
well known and widely utilised in landscape analysis. The revival and present wide acceptance of the
two-stage concept are due largely to Linton and
Budel.
¨
Linton’s contributions to this state of affairs flow
from his analysis of tors. The word ‘tor’ Ž torr, twr,
turris . means a tower and, in Britain, has long been
C.R. Twidaler Earth-Science ReÕiews 57 (2002) 37–74
used of isolated, steep-sided, bare blocky hills Aabout
the size of a houseB ŽLinton, 1952.; though whether
a cottage or a ducal mansion is not clear ŽFig. 9..
Linton categorically dismissed the possibility of the
granite outcrops of Dartmoor having been shaped by
weathering at the surface. Instead, he appreciated the
effectiveness of subsurface moisture attack, and attributed the tors of Dartmoor to Aa two-stage processB ŽLinton, 1955, p. 472..
His papers ŽLinton, 1952, 1955. on the tors of
Dartmoor and other parts of Britain and western
Europe, formalised and provided the essential scientific underpinning for the two-stage ideas that were
in wide circulation at the time, though without referring to the pioneers of the concept. They did much to
establish the two-stage concept as it applied to tors
and boulders in granite, though not in all quarters.
For example, in his well-known text, Holmes Ž1965,
pp. 609–617. accepted deep weathering and the
two-stage mechanism as they applied to ‘tors’, but
he outlined an epigene and multicyclic explanation
for the much larger African inselbergs with which he
had long been familiar. Citing Dixey’s Ž1942. and
King’s Ž1950, e.g.. papers on planation surfaces he
stated:
With each major uplift a new cycle of erosion
started at the coast, encroaching on the pediment
49
of an earlier cycle . . . advancing up the rivers and
their tributaries, wearing back the escarpments,
but leaving massive bastions and projections behind to be slowly worn into groups of inselbergs
and eventually into isolated peaks . . . ŽHolmes,
1965, pp. 612–613..
Like King Ž1949. before him, and in spite of
citing Falconer’s account of Nigerian inselbergs,
Holmes Ž1965, p. 610. could not accept a similar
mechanism for the much larger African residuals. He
was not alone in this, for as astute, innovative and
open-minded a geomorphologist as Lewis Ž1955, p.
483., commenting on Linton’s thesis that tors are
due to differential deep weathering stated: AI was
inclined to think he wLintonx was overdoing the deep
weathering process . . . B, but he was convinced on
seeing the field evidence.
4.6. Budel
¨ and A double planationB
Linton’s expositions on deep weathering referred
mainly to granitic terrains Žbut see Linton, 1964. and
to residuals. With the concept of Doppelten EinebŽ1957,
nungsflachen
or ‘double planation’, Budel
¨
¨
1977. not only contributed much corroborative evidence for, and a valuable refinement of, the basic
two-stage hypothesis, but he argued its applicability
Fig. 9. Haytor, a typical tor on Dartmoor, southwestern England.
50
C.R. Twidaler Earth-Science ReÕiews 57 (2002) 37–74
to the analysis of landscapes; to such good effect that
for many the concept is now inextricably linked with
his name Že.g., Jones, 1999, pp. 17–18.. Budel
¨ envisaged that given tectonic stability, weathering and
river action would together produce a planation surface. A more-or-less thick regolith develops beneath
this surface. In many places, the base of the regolith
is marked by an abrupt transition into relatively fresh
rock: the weathering front, Tiefenfront or Verwitterungs-Basisflache.
The front is irregular, if only
¨
because the progress of weathering varies according
to the character of the country rock, but two surfaces
are simultaneously developed, one at ground level,
and the other at the base of the regolith shaped by
moisture held in the weathered mantle ŽFig. 10; Abb.
5 in Budel,
1957; see also Ollier, 1960..
¨
Budel
emphasised the effectiveness of regolithic
¨
moisture in reducing rocks and producing planate
surfaces. His model accounts for plains transecting
rocks of varied resistance in terms of intensive
weathering and at various scales Žsee also Mabbutt,
1966.. Minor levels within the weathering front at
various scales and due either to weak bedrock, baselevel lowering, climatic fluctuations or, in the deserts
of Western Australia, to localised weathering and
planation by migrating lakes ŽJutson, 1914, p. 156,
1934, pp. 235 et seq., also Fig. 98, at p. 239. are
readily accommodated within the double planation
scheme Žsee, e.g., Busche, 1980.. So are secular
variations in the locus of weathering due either to
climatic or tectonic changes, which result in variations in the level of the water table, and leading to
stepped relief at various scales ŽJessen, 1936; Crickmay, 1974..
In her review of Budel’s
double planation, Bre¨
mer Ž1993. analysed how the regolith developed
beneath the Obere Einebnungsflache
may be evacu¨
ated Žsecond stage of the two-stage mechanism..
Backwearing and downwearing are both theoretically
feasible, depending on the structural setting, and
both types of development are evidenced in the field.
Evidence implying lowering of the regolith in the
Gawler Ranges has been deduced ŽCampbell and
Twidale, 1991; Twidale, 1994, q.v. below.. On the
other hand, isolated plateau remnants capped by
ferruginous or siliceous carapaces persist on Jutson’s
Ž1914. New Plateau. In such structural situations,
scarp retreat is inherently probable, both in terms of
general theory Že.g., Tricart, 1957. and given the
consistent inclinations of slopes bordering remnants
of varied dimensions Že.g., King, 1942; Fair, 1947..
Bremer Ž1993, p. 190. also broached the question
of the origin of inselbergs in the context of Budel’s
¨
model, suggesting that AInselbergs are not explained
by slope retreat or as erosional remnants due to hard
rockB. Rock resistance reflects not only composition
and physical hardness, but also, and especially, fracture density and accessibility to circulating ground-
Ž1957. diagram showing double planation: Ži. initial surface, Žii. development of regolith and weathering front, and Žiii.
Fig. 10. Budel’s
¨
lowering of plain and water table, and development of lower weathering front.
C.R. Twidaler Earth-Science ReÕiews 57 (2002) 37–74
waters. In Budel’s
own diagram ŽFig. 10., the weath¨
ering front is depicted as irregular, with a topography
already developed, as it is in reality in some areas.
To cite a specific example, the Early Cretaceous
etch landscape of the Gawler Ranges, in the arid
interior of South Australia, is one of domes and
51
fracture-controlled valleys developed on a mass of
Mesoproterozoic silicic effusive volcanics. Though
banks of columnar joints are prominent throughout,
weathering patterns and the resultant bornhardt landscape are due to the exploitation of orthogonal systems of steeply inclined fractures ŽFig. 11a. and
Fig. 11. Ža. Diagram of fracture-defined bornhardts in the central Gawler Ranges, South Australia, and Žb. view of bornhardts near Mt Nott,
southern Gawler Ranges.
C.R. Twidaler Earth-Science ReÕiews 57 (2002) 37–74
52
associated sets of sheet fractures. Many of the domical hills are bevelled and the massif is dominated by
the Nott Surface, a summit surface of etch type
ŽCampbell and Twidale, 1991; Fig. 11b.. The regolith was stripped during Neocomian–Aptian times
and the resultant debris, including dacitic corestones,
was deposited in the Eromanga Basin, where it is
now known as the Mt. Anna Sandstone ŽWopfner,
1969, p. 152.. The contained corestones are smaller
low in the sequence than at higher levels. This is the
reverse of the distribution found in a standard weathering profile and suggests that the regolith was eroded
in layers, i.e., the surface was lowered, rather than
being reduced by scarp retreat, which would have
resulted in deposits containing corestones of mixed
size.
5. Some difficulties and implications
5.1. What is A subsurfaceB?
The two-stage mechanism has found wide acceptance in respect of granitic forms and is also applicable in other lithological environments. But consideration of the depth at which etching occurs, and of
possible origins of residuals, in particular, introduces
conceptual and terminological complications. The
question can be considered by examining first some
minor forms and then certain major landscape features.
‘Pitting’ is a term used to describe surfaces that
are rough as a result of differential weathering at the
crystal scale ŽTwidale and Bourne, 1976; Fig. 12.. It
is demonstrably a two-stage form for it can be
revealed by clearing the regolith and is developed on
recently exposed surfaces. It represents the first stage
in the breakdown of the country rock. In granite, for
instance, mica and feldspars are attacked by moisture
at the base of the soil cover and are converted to
clay, while the quartz and some feldspar phenocrysts
resist weathering and remain upstanding. It is well
developed in crystalline rocks, which include minerals of contrasted susceptibility to water attack, but is
also found on limestone where crystal boundaries
and cleavages are preferentially weathered ŽFig. 5c..
Several early workers Že.g., Scrivenor, 1931, p. 137;
Roe, 1951. recorded such surfaces, some of them
quite spectacular, with feldspars projecting more than
a centimetre. They did not, however, use the term
pitting to describe it and they did not attribute it to
subsurface weathering. In some instances, pitting
develops only a few centimetres below the ground
surface, yet it is indubitably of etch and two-stage
character.
Core-boulders are also etch forms and the resultant boulders develop in two stages. The perimeters
Fig. 12. Pitting on granite boulders at Mt Bundey near Darwin, Northern Territory, Australia.
C.R. Twidaler Earth-Science ReÕiews 57 (2002) 37–74
of slightly rounded blocks ŽFig. 13a. are as much
part of the weathering front as those of a perfectly
rounded corestone. The breakdown of sheet structure
after exposure to produce a blocky veneer and the
formation of a nubbin ŽFig. 13b and c. involves a
complex weathering front, but the development took
place in the subsurface, albeit at shallow depth at the
base of a thin regolith.
Waters penetrating along steeply inclined joints in
granite have produced acicular forms and towers in
granitic terrains, for instance, in the Sierra Nevada of
California, around Mt. Whitney and, at a smaller
scale, in the Cathedral Rocks of the Yosemite Valley; in the Crow Tors of Wyoming and Silent City of
Rocks, Idaho; in the Organ Mountains of New Mexico; and in The Needles, South Dakota Že.g., Bateman and Wahrhaftig, 1966; Cunningham, 1969, 1971;
Seager, 1981.. Similar columns or towers are also
found in other lithological settings, in sandstone or
conglomerate plateaux, as in Monument Valley, Utah,
the Parana valley of southern Brazil and the Roper
River basin of the Northern Territory, and in conglomerate in the Meteora district of central Greece,
in parts of the Pyrenees Že.g., Barrere,
` 1968. and in
the Olgas massif of central Australia ŽTwidale and
Bourne, 1978.. Such fracture-defined columns and
towers, whether exposed in escarpments and valley
sides or in massifs, represent extensions of a weathering front. They evolved just below the land surface, and not necessarily at the base of a thick
regolith, for later erosion has in many places outpaced weathering, yet they are, surely, etch and
two-stage forms. Etch features evolve just below the
surface as well as at depth, and beneath massifs as
well as plains.
The extent and context of the problems arising
from forms originating at different locations in the
landscape can be illustrated by a consideration of
towerkarst ŽTurmkarst, karst a` tourelles . in limestone terrains. Limestone towers are convergent
forms for they evolve in various ways. First, and as
cited earlier Žq.v.., in some areas limestone towers
and domes originate deep below the land surface as
gigantic prongs, resulting from differential, presumably fracture-controlled, subsurface moisture attack.
They are classic etch forms. Second, and by contrast
with features of deep-seated origin, but like their
counterparts in sandstone and conglomerate, some
53
towers are due to the exploitation of major steeply
inclined fractures ŽRichardson, 1947; Brook and
Ford, 1978; Twidale and Centeno, 1993; Fig. 14a..
That such exploitation took place below the surface
of massifs or plateaux is strongly suggested by the
occurrence of incipient towers beneath palaeosurface
remnants. The vertical zonation of cave systems
preserved within residual towers suggests a phased
development Že.g., Sweeting, 1950; Lehmann, 1954;
Jennings, 1963; Drogue and Bidaux, 1992..
Whether domical hills or towers emerge from
such exploitation is a function of the spacing of open
steeply inclined fractures, and this in turn varies with
structure and stress, plus duration of attack. Most of
the karst massifs and towers of Perlis, West Malaysia,
for example, are developed on synclinal troughs in
Permian limestone, zones in which the fractures are
tight and virtually impenetrable to meteoric waters
ŽJones, 1978.. The limestones of the Merapoh area
occur in regional synclines and are, in addition,
metamorphosed to marble ŽRichardson, 1950.. Open
fractures are quite widely spaced and rounding of the
upper corners of fracture-defined blocks initially produced squat domes Ž Cupolakarst . rather than comparatively tall towers. Subsequent basal attack, undermining and collapse have transformed some
domes into towers ŽFig. 14b and d.. Evidence of
such scarp-foot solution includes the cliff-foot caves,
basal notches or swamp slots ŽFig. 14c., the bedrock
limits of which demonstrably extend several metres
below plain level and are found around much of the
basal perimeter ŽLehmann, 1954; Sweeting, 1958;
Corbel, 1959a; Wilford and Wall, 1965; Jennings,
1976; Twidale, 1987a; but see also MacDonald,
1976.. Stages in the conversion of domical hills to
residuals with some or all flanks undermined, collapsed and precipitous can be seen in many areas
Žsee Fig. 14b.. Such a mechanism accounts not only
for the conversion of limestone domes to towers, but
also for the local distribution of the two types ŽVerstappen, 1960; Sweeting, 1990.. This is the third
mode of origin, involving basal subsurface attack on
exposed masses.
5.2. Contrasts in rates of weathering on exposed and
coÕered surfaces
The morphology of a few, but unusual, minor
forms has been attributed to the contrast in rates of
54
C.R. Twidaler Earth-Science ReÕiews 57 (2002) 37–74
C.R. Twidaler Earth-Science ReÕiews 57 (2002) 37–74
weathering on exposed and relatively dry rock surfaces beneath a Žmoist. soil cover. Many rock basins,
for example, are initiated as saucer-shaped depressions on platforms ŽFig. 15a. and on the flattish
upper surfaces of fracture-defined blocks. Near the
edges of such blocks the regolith falls away so that
bare rock is exposed. The central areas retain the soil
and the contained moisture. Weathering proceeds
more rapidly in the latter areas than at the margins,
which thus come to form annular rims enclosing
depressions: what have been termed rock doughnuts
ŽBlank, 1951; Fig. 15b.. Rock levees ŽFig. 15c. also
develop in this manner ŽTwidale, 1988; Vidal Romani and Twidale, 1998, pp. 285–288, 300–301..
Contrasts in rates of weathering have been invoked
also in partial explanation of some landform assemblages, such as flared or concave slopes ŽTwidale,
1962. and the stepped granitic topography of parts of
the southern Sierra Nevada of California ŽWahrhaftig, 1965., though variations in fracture density and
reinforcement or positive feedback mechanisms have
also played their part, here as elsewhere.
Rock doughnuts and ‘fonts’, or benitiers, in sandstone on the west coast of Eyre Peninsula ŽSouth
Australia. also have been interpreted as ‘part-free’
forms ŽTwidale and Campbell, 1998; Fig. 15d..
Flared slopes and rock basins are well developed in
rhyolitic tuff in the Grant County City of Rocks,
New Mexico ŽMueller and Twidale, 1988.. Vertical
pipes analogous to the orgues geologiques
of chalk
´
terrains are well represented in siliceous strata, from
the high rainfall tropics ŽUrbani, 1986. to arid lands
ŽMilnes and Twidale, 1983., etc.—many minor forms
are of two-stage origin and have a considerable
lithological and climatic range Žsee, e.g., Campbell
and Twidale, 1995..
5.3. Contrasts between deep and shallow weathering
Subsurface scarp-foot weathering is especially
pronounced in readily soluble carbonate terrains, but
is not restricted to such environments. Frequent ac-
55
cessions of meteoric waters and the relative abundance of biota and organic remains at and just
beneath the surface also ensure that shallow subsurface groundwaters are, however, the most aggressive
and effective in chemical weathering. Paton Ž1964;
see also MacDonald, 1967., for instance, has interpreted the karst towers of West Malaysia as monadnocks de position located on divides, and preserved
because meteoric waters are less effective in dissolving the country rock than are river and swamp waters
with their acquired biota and chemicals. Thus, as
they travel over and just beneath the surface, soil
moisture, shallow groundwaters and streams become
more aggressive and more rapidly dissolve the carbonate country rock so that the headwater zones are
preserved while those downstream are reduced. It
might well be asked whether location on divides is
cause or effect, but the suggestion highlights the
perceived significance of water quality in weathering.
Aggressive subsurface moisture attack is commonly responsible for the piedmont angle, the sharp
break of slope between hillslope and piedmont ŽTwidale, 1967.. Around some granite inselbergs Že.g.,
Kokerbin Hill in the southern Yilgarn Craton of
Western Australia., undermining and collapse of hillslopes has resulted in the slippage of sheet structures,
thus lowering the slope and reducing the size of the
dome, without, however, modifying its shape. The
collapse and steepening of limestone slopes has radically changed the profiles of the residuals. Collapse
following undermining due to subsurface scarp-foot
weathering and erosion is also evidenced at the base
of the arkosic Ayers Rock in central Australia. The
bevelled summit of Ayers Rock ŽFig. 16a. is an etch
surface of latest Cretaceous age ŽHarris and Twidale,
1991.. The steep sides of the inselberg have developed and been exposed in at least two phases of
scarp-foot subsurface weathering during the Cainozoic: the earlier resulting in a row of gaping-mouth
caves and breaks of slope 35–60 m above present
plain level, the later in fretted and flared basal slopes
Fig. 13. Ža. Blocks of dolerite, little rounded by weathering, exposed in a road cutting near Umtata, Eastern Cape Province, South Africa. Žb.
Slope on Enchanted Rock, central Texas, showing sheet structure broken down into angular blocks. Žc. Nubbin, or block- and
boulder-strewn inselberg, Naraku, north of Cloncurry, northwestern Queensland.
56
C.R. Twidaler Earth-Science ReÕiews 57 (2002) 37–74
C.R. Twidaler Earth-Science ReÕiews 57 (2002) 37–74
57
Fig. 14. Ža. Incipient dolomite towers exposed in the flanks of the Arroyo de Valdecabras, near Cuenca, central Spain. Note the flared sides
of some of the columns. Žb. Towerkarst near Ipoh, West Malaysia. Note steepened sidewall of tower in central middle distance. ŽJ.N.
Jennings.. Žc. Swamp slot near Ipoh, West Malaysia. Žd. Diagram illustrating conversion of cone- to tower-karst through basal sapping. At a
and b, the development of swamp slots at and just below the surface causes undermining, followed by collapse and steepening of slopes
ŽTwidale, 1987a..
4–5 m high ŽTwidale, 1978b; Fig. 16b–d.. At each
stage, the exposed form was modified by subsurface
scarp-foot weathering, which caused undermining
and collapse of slopes.
The contrasted effectiveness of deep and shallow
regolithic weathering also finds expression in the
morphology of granitic residuals. The bornhardt is
the basic form and rapid deep stripping of the regolith results in such domical hills being exposed,
but nubbins and koppies reflect the difference between subsurface weathering in warm humid environments and weathering of a partly buried, partly
exposed, mass ŽTwidale, 1981..
On the other hand, the deep and prolonged weathering that is an intrinsic feature of much two-stage
development carries different implications for landscape development. The regolith holds water, the
most important single weathering agency. Soluble
products of weathering are translocated within the
regolith by circulating shallow Žvadose. groundwaters. Where the soluble products of weathering are
evacuated out of the system, volume decrease may,
according to Trendall Ž1962. lead to the settling,
compaction and general lowering of the surface.
Thus, there is an argument for considering shallow, as well as deep, long-term as well as ephemeral,
two-stage development. It may also be useful to
distinguish between etching beneath plains, as in
Falconer’s and Budel’s
models, and similar pro¨
cesses operating in massifs and uplands where, because of lower water tables and through-drainage,
descending meteoric waters are only briefly in contact with bedrock. Certainly the potency of shallow
groundwaters calls for particular consideration.
At depth, weathering may be long continued, and
stripping of the regolith long delayed, frequently
58
C.R. Twidaler Earth-Science ReÕiews 57 (2002) 37–74
Fig. 15. Ža. Saucer-shaped depressions on a newly exposed granite platform, Kwaterski Rocks, north of Minnipa, northwestern Eyre Peninsula, South Australia. Žb. Rock
doughnut in granite, high on the slope of Enchanted Rock in the Llano, central Texas. The basin is crucial to its development. Water from the weathered granite or grus drained
into the basin. The bedrock beneath this comparatively dry grus was not weathered as rapidly as was that a few centimetres farther from the basin. Thus, an annular rim of rock
developed around the depression. Žc. Rock levees bordering a gutter on the slopes of Domboshawa, a granitic bornhardt near Harare, Zimbabwe. Žd. Doughnuts and fonts in
sandstone, Talia, west coast of Eyre Peninsula, South Australia. Like the doughnut illustrated in Fig. 15b each of the forms originated around a rock basin into which drained
water from the adjacent regolith or, as here, beach sand. As contrasted weathering continued, the rims and basins were left higher and higher in the local relief, converting
doughnuts into fonts, arbitrarily when the height becomes greater than the maximum diameter.
C.R. Twidaler Earth-Science ReÕiews 57 (2002) 37–74
with spectacular results in the form of plains of
extraordinary flatness and considerable extent. For
example, the Meekatharra Plain ŽFig. 17a., an inselberg landscape in the central Yilgarn Craton of
Western Australia, cuts across granite and gneiss,
and is an exposed weathering front formed beneath a
regolith of Triassic–Cretaceous age, lateritic remnants of which remain ŽFig. 17b.. Similarly, the
Bushmanland Surface, which extends from northern
Namaqualand ŽWestern Cape Province. into central
Namibia, South Africa, is cut across granite, gneiss,
sandstone and schist ŽFig. 17c.. It too is of etch type
for duricrusted regolithic remnants are preserved in
places, e.g., near Platbakkies ŽPartridge and Maud,
1987., and is of probable Cretaceous age ŽFig. 17d..
The etch plain, which occupies much of Finland, is
similarly devoid of relief Že.g., Soderman,
1985..
¨
Susceptibility to chemical attack can, however,
compensate for duration of weathering. The flatness
of the Nullarbor Plain ŽFig. 18. has long puzzled
investigators ŽJennings, 1963.. It is not a structural
feature, ‘a single exposed bedding plane’, but what
degradational process could produce such a feature?
The surface, some 200,000 km2 in extent, is eroded
in flat-lying Miocene limestone, but at least 60 m of
section have been removed near the southern or
coastal margin of the plain ŽLowry, 1970; Lowry and
Jennings, 1974.. Weathering Ždissolution. by moisture retained in a thin regolith, of which remnants
survive, may be responsible ŽTwidale, 1990a..
But karst regions are also, and especially, subject
to double planation. In addition to the extreme shallow planation exemplified in the Nullarbor Plain,
deeper water table weathering and erosion at various
levels Že.g., Sweeting, 1950; Jennings, 1963. simultaneously induce solution and collapse to that level
or levels, eventually producing a limestone plain or a
towerkarst landscape like those found in many parts
of the Antilles and Southeast Asia. Such karstic
double planation is surely in train in carbonatedominated sequences, such as that of the western
Murray Basin in South Australia.
5.4. Alternations of weathering and erosion, or
steady-state deÕelopment?
Two-stage development implies periods when
weathering was dominant followed by phases of
59
erosion, and several workers have investigated possible reasons for such alternations. Willis was aware
that changes due to earth movements, climatic variations and river development Žextension and incision.
had impacted on the landscape. Fairbridge and Finkl
Ž1978. examined the evolution of the Yilgarn Craton
of Western Australia in terms of its tectonic and
climatic history, traced drainage adjustments to these
events and identified several alternations of weathering and erosion. This idea was developed into what
is termed a cratonic regime ŽFairbridge and Finkl,
1980., which takes account of tectonic, eustatic and
climatic changes, and is characterised by alternations
of thalassocratic–biostatic phases of deep chemical
weathering and epeirocratic–rhexistatic periods during which the regolith is eroded in part or in whole.
Working in West Africa, Thomas and his colleagues
have linked etch planation to known episodic environmental changes of the Quaternary Že.g., Thomas,
1989a; Thomas and Thorp, 1985., but in many areas,
the reasons for alternations of weathering and erosion remain obscure. The environmental settings of
the many flared slopes developed, exposed and preserved on the inselbergs of southern Australia, for
example, remain conjectural. Could erosion become
dominant not through any environmental change, but
as a result of the progress of weathering such that the
alterites become susceptible to transport when a certain texture or grain size is attained, or could erosion
be triggered by a catastrophic event, such as a local
flood, the effects of which extend in time throughout
the catchment?
However, the many forms that are manifestly of
etch origin have involved weathering and then erosion. The most obvious two-stage forms are those in
which, for various possible reasons, erosion most
recently has outpaced weathering, but where there
are enough remnants of the former cover to demonstrate the likely mechanism responsible for the assemblage. But as Lewis Ž1955. astutely appreciated,
near the surface, the two processes may be active yet
result in no morphological change. Where the two
proceed at equal rates, however, the surface is in
dynamic equilibrium and bedrock forms are produced with little or no evidence of a regolith or of a
weathering front: some etch forms may be difficult
to identify as such. Fortunately, local variations in
rates of weathering and erosion may have caused
60
C.R. Twidaler Earth-Science ReÕiews 57 (2002) 37–74
C.R. Twidaler Earth-Science ReÕiews 57 (2002) 37–74
Fig. 16. Ža. Bevelled summit surface of Ayers Rock, central
Australia. Note also the steep flanks with break of slope about 35
m above the plain and, in the foreground, a rock platform some
800 m from the base of the inselberg. Žb. Break of slope and
associated gaping-mouth caves or tafoni at the southeastern corner
of Ayers Rock. Žc. Fretted base of Rock. Note bevelled crest,
tafoni, and blocks and boulders fallen from slope, and Žd. flared
southern base of Ayers Rock.
patches of regolith and other evidence Žsuch as pitting. of a former regolith to be preserved, thus
allowing reconstruction of the sequence of events
and of the origin of the forms. Thus, the etch origin
of many rock pediments or platforms, such as those
so graphically recorded by McGee Ž1897., is indicated by the patches of thin regolith preserved alongside the bedrock surface and, in granitic terrains, by
small Žcore. boulders.
5.5. Zonation, climatic and lithological?
Two-stage forms are well evidenced in the humid
tropics Že.g., Thomas, 1989a,b; Thomas and Thorp,
1985., where regoliths in the order of 60–200 m are
not uncommon Že.g., Branner, 1896; Scrivenor, 1931;
Ingham and Bradford, 1960; Ollier, 1965; Thomas,
1966.. Particular forms and assemblages, such as the
towers and domes of karst areas and the nubbins of
granitic terrains, are well represented there. Though
the development of karst towers in the Yukon ŽBrook
and Ford, 1978. has focused attention on structural
61
factors, it cannot be denied that towerkarst is best
developed and most widely preserved in the humid
tropics; so much so that extratropical occurrences,
like those of southern Switzerland and southern
Poland Že.g., Gilewska, 1964., are explained in terms
of exhumation or climatic change, just as are granitic
nubbins found in contemporary arid zones ŽOberlander, 1972..
Again, inselberg landscapes comprise not only
extensive plains, but also inselberg remnants, and
there has evolved a suggestion that the latter, and by
implication the whole assemblage, is most commonly associated with the tropical savannas Že.g.,
Krebs, 1942: cited in Hovermann,
1978.. In these
¨
terms, the occurrence of two-stage inselberg landscapes in, say, arid or temperate or cold lands implies climatic change Že.g., Linton, 1955; Budel,
¨
1978..
Yet the assumption that any two-stage form initiated at the weathering front is climatically zonal
must surely be questioned. Etch forms are due to
chemical weathering by shallow groundwaters armed
with chemicals and biota. Such groundwaters are
ubiquitous. Concentrated in the 800 m or so below
the land surface, though detected at depths of up to
10 km ŽKozlovsky, 1987., groundwaters extend beneath each and every part of every continent in either
liquid or solid form. Even in the midlatitude deserts,
they occur at depth. The hydrosphere of which the
groundwater zone is part forms a continuous, though
in places much attenuated, shell. Regoliths are widely
developed and, depending in considerable measure
on the nature of the country rock, clearly defined
weathering fronts also are well represented. It is
topographically differentiated, initially according to
the structure of the country rock, though later with
positive feedback mechanisms taking effect. Thus,
fractures and fracture zones are first exploited to
produce, say, the bedrock projections revealed in
subsurface investigations at Sepang ŽFig. 7., but
once formed, shallow groundwaters flow along the
bedrock surface and into depressions where consequently basal slopes are weathered and steepened.
Discrete sectors of the front are also ubiquitous and,
as Linton Ž1955, p. 472. pointed out, ACore-stones
are not . . . confined to the tropicsB.
Different weathering complexes are dominant in
different climaticrbiotic environments as well as in
62
C.R. Twidaler Earth-Science ReÕiews 57 (2002) 37–74
Fig. 17. Ža. Part of the Meekatharra Plain cut across granite and gneiss, central Yilgarn Block, Western Australia. Žb. A lateritic remnant of the Old Plateau standing above the
New, near Cue, Western Australia. Žc. The Bushmanland Surface in northern Namaqualand ŽWestern Cape Province., South Africa is cut across granite but to the north, transects
gneiss, schist and sandstone. Žd. Silcrete-capped mesa, near Platbakkies, northern Namaqualand ŽWestern Cape Province.. The siliceous capping has been undermined and the
underlying white kaolinised zone eroded to expose the weathering front in gneiss as an etch plain ŽJ.A. Van Zyl..
C.R. Twidaler Earth-Science ReÕiews 57 (2002) 37–74
63
Fig. 18. View over the Nullarbor Plain, underlain by Miocene Nullarbor Limestone which overlies the Eocene Wilson Bluff Limestone.
ŽAdvertiser Newspapers, Adelaide..
different lithological settings. Rates of development
vary Že.g., Corbel, 1959b; Judson and Ritter, 1964..
Regoliths have been evacuated mostly by rivers, but
also in places and at times by glaciers and waves
and, though more rarely and locally, by the wind
Že.g., Peel, 1966.. Some forms, such as bornhardts,
karst towers and pediments, are widely and well
developed in some climatic regions, but, though
scarce and poorly developed, are also found in others: absolute zonality is inherently unlikely. Forms
originating at the weathering front—etch forms and
surfaces—whether pitting, rock basins, corestones or
clefts, scarp-foot embayments or fracture-defined
residual masses, are ubiquitous. The occurrence of
two-stage forms in the present humid and arid tropics and in midlatitude lands is well documented, but
in view of the many remnants of preglacial regoliths
known to have survived passage of ice sheets Že.g.,
Gauthier, 1980; Bouchard, 1985; Fogelberg, 1985;
Grant, 1989., it is not unreasonable to interpret many
of the bedrock plains and associated minor forms of
the high-latitude shields in North America and Scandinavia as etchplains. They are integral components
of all oldlands ŽWilson, 1903; Twidale, 1990a, 1999..
Forms resulting from the etch or double planation
mechanism are developed in a wide range of rock
types ŽCampbell and Twidale, 1995., though they are
most commonly preserved in rocks which are, for
whatever reason, inherently resistant when dry. Thus,
the extensive, if fragmentary, etch surface of central
Australia ŽMabbutt, 1965. is preserved on sandstone
or quartzitic ridge crests in such fold uplands as the
Macdonnell and Davenport ranges. Granite, sandstone and limestone are common hosts to etch forms.
Those developed in weak materials, such as argillites,
are only preserved in particular circumstances. In the
Flinders Ranges of South Australia, for example,
etch plains in shale, mudstone, etc., are preserved Ža.
standing close to local baselevel ŽFig. 19a., Žb. where
covered and protected by a lag of gravel or gibber,
Žc. where buttressed by resistant strata, and Žd. in
favourable structural settings, e.g., in the cores of
deeply eroded anticlines ŽTwidale and Bourne, 1996.;
or where more than one of these conditions obtains.
Two-stage surfaces are developed in coastal as
well as interior locations, for waves and other marine
processes are capable of stripping regoliths and exposing weathering fronts. Thus, many shore plat-
64
C.R. Twidaler Earth-Science ReÕiews 57 (2002) 37–74
Fig. 19. Ža. Etch plain in dipping shale and thin limestone near Rawnsley Bluff, central Flinders Ranges, South Australia. X indicates
exposure of kaolinitic regolith in old piedmont remnant. Žb. Diagrammatic section through Point Drummond, on the west coast of Eyre
Peninsula showing the regolith capped by a small remnant of dune calcarenite and with the weathering front exposed as a rocky shore
platform.
forms in granitic rocks found on the west coast of
Eyre Peninsula are of etch type ŽMolina Ballesteros
et al., 1995; Fig. 19b., as are some of the platforms
developed in mudstone on the California coast Že.g.,
Fig. 4 in Bradley and Griggs, 1976..
5.6. Landscape deÕelopment
Appreciation and awareness of the two-stage development influences how landscapes are interpreted.
First, as signalled by Willis Ž1936, p. 124., the
two-stage model offers an alternative to the multicyclic interpretation of the stepped landscapes that
dominate many parts of the world Že.g., King, 1962;
Crickmay, 1974.. Thus, the landscape of northwest
Queensland has been interpreted in conventional
cyclic terms, albeit with significant exhumed elements ŽTwidale, 1956a.. A lateritic surface of mid
Tertiary age has been dissected and on the divide
between the exoreic and endoreic drainage the resultant Late Cainozoic fluvial plain is extending at the
expense of the duricrusted Kynuna Plateau. The
weathering front preserved beneath the latter is coincident with that of the adjacent plain, so that in the
vicinity of the Kynuna Plateau the younger plain
could be of etch type.
C.R. Twidaler Earth-Science ReÕiews 57 (2002) 37–74
Second, two-stage forms are well named for they
have two ages, one referring to the period of subsurface weathering and the preparation of the weathering front, the other to the period of exposure of the
front as part of the land surface ŽTwidale, 1987b..
This is of some significance in palaeogeographical reconstructions ŽTwidale, 2000.. Thus, that the
Nott Surface of the Gawler Ranges ŽFig. 11b. is
demonstrably an etch surface, and an untere Einebnungsflache,
proves the former existence of an obere
¨
Einebnungsflache
beneath which it evolved by dif¨
ferential subsurface weathering. There is no known
sign of this Beck Surface in the present landscape
ŽCampbell and Twidale, 1991., yet its former existence is undeniable ŽTwidale, 2000..
The term ‘two-stage’, though apt, is, in many
instances, a simplified description of the mechanism,
for the origin of many such forms can be traced back
much further in time, in some instances, to the
formation of the bedrock in which they are shaped.
Thus, the bornhardts of the Gawler Ranges were
produced by etching in the Triassic and Jurassic
Žstage 1., and exposed as landforms in the Early
Cretaceous Žstage 2.. The etching exploited orthogonal fracture systems formed by regional stress soon
after the volcanic rocks had cooled and consolidated
almost 1600 Ma. This bornhardt landscape is a multistage rather than simply a two-stage feature ŽTwidale and Vidal Romani, 1994., though it is the
etching and subsequent stripping of the regolith that
in an immediate sense are responsible for the contemporary landscape.
Third, the recognition of surfaces as of two-stage
and etch origin has confirmed the suggestion that
some have been stable since exposure while all
around has been weathered and eroded. For instance,
surfaces dimpled by rock basins and scored by gutters are typical etch forms. The crest of Yarwondutta
Rock, on northwestern Eyre Peninsula, is of this type
ŽFig. 20a. and it shows that the surface has remained
essentially unchanged since exposure, while successive alternations of subsurface weathering and erosion have produced a stepped inselberg ŽFig. 20b–d.,
lower and lower plains, and increasingly greater
local relief amplitude ŽTwidale, 1982c; Twidale and
Bourne, 1975b..
Fourth, and as Wayland Ž1921., Knopf Ž1924.,
Crickmay Ž1932, 1976., Willis Ž1936. and Horton
65
Ž1945. appreciated, examples of etched and multiphase stepped residuals, such as Yarwondutta Rock,
demonstrate that destructive geomorphological activities are unevenly distributed over the land surface.
Wayland Ž1921, p. 40., for example, observed that:
the excavation of the lower ground may accelerate
upon that of the hills . . . thus may the turtle-backs
of the past become the inselberg hills of the
present.
In similar vein, Knopf Ž1924, p. 637. pointed out
that in the Appalachians some divides are Aout of
reach of erosionB and Amore-or-less immune from
destructional agencies of the present cycleB Žp. 642..
Clearly, increases in relief amplitude that are independent of tectonism have developed, assisting in the
preservation and persistence of very old palaeoforms
ŽTwidale, 1976, 1991, 1994..
Fifth, the concept of double planation impinges
on the interpretation of anomalous drainage patterns.
As Budel
¨ pointed out ŽBremer, 1993, p. 190. untere
Einebnungsflachen
are independent of surface drai¨
nage Žthough a subterranean pattern may well evolve
on the topographically differentiated weathering front
or lower surface.. Erosional lowering may result in
the drainage pattern of the original surface being let
down on to the weathering front, where it may well
be transverse to structure: inherited drainage ŽCotton,
1948, p. 56.. Several anomalous rivers, but notably
the Finke, which drain the Macdonnell Ranges and
adjacent areas of central Australia, flow toward Lake
Eyre across a formerly duricrusted surface, now
partly eroded to form an etch plain ŽMabbutt, 1965.
and across folds, faults and a multitude of rock
types. The transverse courses of such rivers can be
explained partly as inherited from a regolithic surface, partly as due to AautosuperpositionB, or stream
persistence and valley impression ŽOberlander, 1965;
Twidale, 1972..
5.7. Questions of terminology
Forms and surfaces, which originate at the weathering front and later have been exposed as a result of
the stripping of the regolithic cover, are widely
distributed in the Earth’s landscapes. The mechanism
was recognised early, at least in the context of the
development of Geomorphology, and is increasingly
66
C.R. Twidaler Earth-Science ReÕiews 57 (2002) 37–74
Fig. 20. Ža. Dimpled granite platform on the summit of Yarwondutta Rock, near Minnipa, Eyre Peninsula, South Australia. ŽC. Wahrhaftig.. Žb. The residual viewed from the
north. The reservoir at the northern margin is seen, in close up, Žc. wherein flared slopes are naturally exposed Žbackground., and revealed in the excavation. Žd. The stepped
northwestern slope of Yarwondutta Rock, due to exposure of the residual by episodic lowering of the adjacent plains.
C.R. Twidaler Earth-Science ReÕiews 57 (2002) 37–74
accepted as providing the key to understanding significant spatial and temporal aspects of landscape
and landform development. Yet there is no agreement as to what the concept ought to be called.
The term Doppelten Einebnungsflachen
is disad¨
vantageous as a general description of the mecha-
67
nism because of the connotations attaching to the
term ‘planation’. In a geomorphological sense, it
suggests both an event, a phase of lowering, and its
result, the formation of a plain or plane surface, i.e.,
a surface of low relief. In nature, however, the
weathering front may not be flat so that many two-
Fig. 21. Ža. Part of the group of granite pillars known as Murphy Haystacks, western Eyre Peninsula, South Australia. Note the
concave-inward, or flared, sidewalls. Žb. Domical residuals in sandstone, western George Gill Range, central Australia. The platforms at
midslope are erosional and may be due to subsurface weathering associated with past water table fluctuations.
68
C.R. Twidaler Earth-Science ReÕiews 57 (2002) 37–74
stage etched landscapes are topographically differentiated. Some etch surfaces are flat, but many wellknown forms of this type are not planate, so that
Budel’s
concept most directly refers to admittedly
¨
notable features, which were, however, developed in
particular circumstances. Otherwise, ‘double-planation’ is an admirably evocative term describing the
mechanism responsible for some notable examples
of the genre: but only some.
‘Two-stage’ is equally meritorious and evokes the
mechanism involved, and though many forms are
demonstrably multistage in origin, the two most
critical stages in landform development are those
involving subsurface initiation and subsequent exposure.
The word ‘etch’ suggests the character of the
processes involved in their formation, as does A la
corrosion chimiqueB ŽTwidale, 1990b., though the
latter is tautological. Would the French verb graÕer
—to engrave, fulfil a similar purpose? All these
terms have the advantage of applying to forms at any
scale. The prefix ‘crypto’ and the adjective ‘covered’
have been suggested in the karst context while Zwittkovits Ž1966., possibly taking his cue from Miller
Ž1953., referred to covered Žkarst. forms as subkutan, a term which has the advantages of being readily translated into French Ž sous-cutane´. and English
Žsubcutaneous., and of allowing forms which originate within the regolith, forms, such as corestones,
logically to be termed AintracutaneousB ŽTwidale,
1987b.. All are useful terms, particularly those which
distinguish between process and mechanism.
The term ‘tor’ has also given rise to some confusion. Most writers have used the word to imply a
small steep-sided hill. Several, though not all workers Že.g., Williams, 1936; Mabbutt, 1952; Thomas,
1965., like their colleagues earlier working in southwestern England, applied the term ‘tor’ to what are
clearly boulders as sedimentologically defined, i.e.,
detached and rounded rock masses with a diameter
of at least 256 mm Žabout 10 in.. and no formal upper limit ŽLane et al., 1947.. Linton Ž1955.
expressed a preference for Acore-stoneB over
Scrivenor’s Acore-boulderB Žq.v.. because, he argued, ‘stone’ carries no size limitation, whereas
‘boulder’ does. In fact, and as stated, no upper limit
is implied by the term ‘boulder’. Some authors
varied in their usage implying a boulder in some
papers, but a steep-sided hill elsewhere. Another
difficulty is that the castellated remnants called ‘tors’
in southwestern England and in New Zealand Že.g.,
Cotton, 1917, p. 288; Raeside, 1949. are in other,
and especially African, settings referred to as castle
koppies.
But the critical difference between a boulder and
a tor or tower is not size. It is that the former is
detached and separated from the main rock mass by
a zone of weathered rock, whereas the latter remains
in physical continuity with it, with the rock extending without break from the substrate into the residual
or separated only by a fracture. Linton’s diagram
depicted a tor, for the rock mass of the residual is in
essential continuity with the underlying rock, but the
residual was shown as comprising blocks some of
which are rounded.
Contiguity is a significant characteristic distinguishing boulders from small domes, pillars and tors
or koppies. In foliated gneisses, tabular residuals
known variously as penitent rocks, monkstones,
ŽAckermann, 1962., and
tombstones, Bussersteine
¨
larger residuals Žwhat some have called Gefugerelief
¨
—Turner, 1952., are shaped in the shallow subsurface, but, unlike corestones, they remain in physical
continuity with the underlying bedrock. The larger
rather squat pillars in granite ŽFig. 21a. constitute a
genetic link between small projections and bornhardts Že.g., Twidale and Campbell, 1984; but see
also Brajnikov, 1953.. Similarly, beehive forms or
ŽTwidale, 1956b., found in monsoonal
Bienenkorbe
¨
north Queensland have their larger scale counterparts
both in the George Gill Range ŽFig. 21b. of central
Australia and the Bungle Bungle Ranges of the
eastern Kimberleys of Western Australia ŽYoung,
1986.. On the other hand, a large isolated rounded
mass like The Leviathan, an ellipsoidal mass measuring 33 m long by 21 m wide by 12 m high, and
standing in the Mt. Buffalo massif of southeastern
Victoria ŽDunn, 1908., is a boulder because it is
detached.
6. Conclusions
Many of the investigations, which led to the
interpretation of landscapes in terms of two-stage
development, were carried out in the tropics, but the
C.R. Twidaler Earth-Science ReÕiews 57 (2002) 37–74
concept is of much wider, and indeed global, application. Many, perhaps most, destructional forms
originate in the subsurface in greater or lesser measure. The recognition and understanding of minor
etch forms were readily achieved, for evidence is
widely exposed. The application of the concept to
major features, however, called for imagination and
has been complicated by considerations of scale
Ždepth of weathering and consequent scarcity of
exposure, plus a failure to link shallow and deep
developments., time Žboth absolute, and the relative
rates of weathering and erosion. and the mechanisms
involved Žthe effects of weathering in time, the mode
of regolith removal., and space Žconfusion of absolute zonality as opposed to prevalence of occurrence..
Nevertheless, the two-stage concept is now widely
accepted and is an invaluable tool in the analysis and
interpretation of landscapes in their spatial and temporal settings.
The two-stage concept has a lengthy history, with
published expositions relating to minor forms dating
from more than 200 years ago and to major features
and landscapes from more than a century. In terms of
landscape evolution it is properly associated with the
names of Wayland, Linton and Budel.
But others
¨
prepared the way, with Hassenfratz, MacCulloch,
Logan, Kingsmill, and Pumpelly in the van and
Falconer and Willis contributing crucial insights concerning the sequence of events implied and, the
processes involved, in two-stage evolution. Hassenfratz and Falconer, in particular, provided succinct
accounts of the essential mechanism. The etch or
two-stage concept did not, as is commonly implied,
originate with Wayland.
Analysis of how the two-stage concept evolved
confirms the suggestion that in science the credit
frequently—and perhaps deservedly—goes to the
person who assembles evidence and argument
wherewith to sustain the idea, to demonstrate its
viability pro tempore, and not to the one who first
thought of it; ingenuity in devising tests for ideas is
almost as important as the ideas themselves. While
praising those who convinced the world, however,
sight surely ought not to be lost of those who had the
imagination and courage—for people are afraid of
unfamilar ideas and the unconventional can attract
obloquy—to interpret familiar forms in unaccustomed terms: it is salutary to bear in mind that
69
AWhat is now proved was . . . once only imaginedB.
Whitehead is said to have warned that a science,
which hesitates to forget its founders is lost, and
though the justification for such a statement is obvious, credit for fertile ideas ought surely to be directed to those to whom it is due. This is especially
so of ideas like the two-stage and etch concepts,
which have changed geomorphological perceptions
of the world. For instead of seeking explanations for
the varied forms of the land surface only in epigene
processes, we must perforce also look beneath the
surface and examine and consider the processes operating at the base of, and within, the regolith.
Acknowledgements
The writer thanks various friends and colleagues,
particularly, Professor Dr. Hanna Bremer of Wilhelmsfeld, Germany, and Dr. Jennie Bourne, Adelaide, for a critical reading of the paper in draft form
and for helpful suggestions; Professor Brian Skinner
of Yale University for directing me to Aaron Waters’
informative and inspiring obituary of Bailey Willis;
and Professor Philip Withers ŽPerth., Professor Charles Hutchison, and Dr. H.D. Tjia ŽKuala Lumpur. for
invaluable assistance in tracing certain references.
Professor Ian Douglas and an anonymous referee
offered constructive suggestions, which are much
appreciated. The views expressed are, however, his
own.
Much of the Australian field work on which this
review is based was supported over the years by
various grants from the Australian Research Committee.
References
Ackermann, E., 1962. Bussersteine-Zeugen
vorzeitlicher Grund¨
wasserschwankungen. Z. Geomorphol. 6, 148–182.
Agassiz, L., 1865. On the Drift in Brazil, and on decomposed
rocks under the Drift. Am. J. Sci. Arts 40, 389–390.
Ansted, D., 1859. An Elementary Course of Geology, Mineralogy
and Physical Geography. Van Voorst, London, 584 pp.
Barrere,
` P., 1968. Le relief des Pyrenees
´ ´ centrales occidentales. J.
Etud. Pau-Biarritz 194, 31–52.
Bateman, P.C., Wahrhaftig, C., 1966. Geology of the Sierra
Nevada. In: Bailey, E.H. ŽEd.., Geology of Northern California. Calif. Div. Mines Geol. Bull. 190, 107–172.
70
C.R. Twidaler Earth-Science ReÕiews 57 (2002) 37–74
Birot, P., 1958. Les domes
crystallins. Mem. Doc. Cent. Nac.
ˆ
Rech. Sci. 6, 7–34.
Blank, H.R., 1951. ‘Rock doughnuts’, a product of granite weathering. Am. J. Sci. 249, 822–829.
Bornhardt, W., 1900. Zur Oberflachengestaltung
und Geologie
¨
Deutsch Ostafrikas. Reimer, Berlin, 595 pp.
Bouchard, M., 1985. Weathering and weathering residuals on the
Canadian Shield. Fennia 163, 327–332.
Boye,
artificiel d’un dome
´ M., Fritsch, P., 1973. Degagement
´
ˆ
crystallin au Sud-Cameroun. Trav. Doc. Geogr.
Trop. 8, 69–94.
´
Bradley, W.C., Griggs, G.B., 1976. Form, genesis, and deformation of central California wave-cut platforms. Geol. Soc. Am.
Bull. 87, 433–449.
Brajnikov, B., 1953. Les pains-de-sucre du Bresil:
´ sont-ils racines?
´
C. R. Somm. Bull. Soc. Geol.
´ Fr. 6, 267–269.
Branner, J.C., 1896. Decomposition of rocks in Brazil. Geol. Soc.
Am. Bull. 7, 255–314.
Bremer, H., 1993. Etchplanation, review and comments of Budel’s
¨
model. Z. Geomorphol., Suppl. 92, 189–200.
Brook, G.A., Ford, D.C., 1978. The origin of labyrinth and tower
karst and the climatic conditions necessary for their development. Nature 275, 493–496.
Buckland, W., 1839. On the action of acidulated waters on the
surface of the chalk near Gravesend. Br. Assoc. Adv. Sci.,
76–77.
Budel,
J., 1957. Die ADoppelten EinebnungsflachenB
in den
¨
¨
feuchten Tropen. Z. Geomorphol. 1, 201–228.
Budel,
J., 1977. Klima Geomorphologie. Borntraeger, Berlin, 304
¨
pp. wFischer, L., Busche D. Žtransl.., 1981. Climatic Geomorphology. Princeton Univ. Press, Princeton, NJ, 433 pp.x.
Budel,
J., 1978. Das Inselberg-Rumpfflachen
relief der heutigen
¨
¨
Tropen und das Schicksal seiner fossilen Altformen uin anderen Klimazonen. Z. Geomorphol., Suppl. 31, 79–110.
Busche, D., 1980. On the origin of the Msak Mallat and Hamadat
Manghini escarpment. In: Salem, M.J., Busrewil, M.T. ŽEds..,
The Geology of Libya, vol. 3. Academic Press, London, pp.
837–848.
Caillere,
de la
` S., Henin, S., 1950. Etude de quelques alterations
´
phlogopite a` Madagascar. C. R. Seances
Acad. Sci. ŽParis.
´
230, 1383–1384.
Campbell, E.M., Twidale, C.R., 1991. The evolution of bornhardts
in silicic volcanic rocks in the Gawler Ranges, South Australia. Aust. J. Earth Sci. 38, 79–93.
Campbell, E.M., Twidale, C.R., 1995. Lithologic and climatic
convergence in granite morphology. Cuad. Lab. Xeol. Laxe
20, 381–403.
Clarke, J.D.A., 1994. Geomorphology of the Kambalda region,
Western Australia. Aust. J. Earth Sci. 41, 229–239.
Commander, D.P., ŽComp.., 1989. Hydrogeological Map of Western Australia. Scale 1:2,500,000. Geological Survey of Western Australia, Perth.
Corbel, J., 1947. Observations sur le karst couvert en Belgique.
Bull. Soc. Belg. Etud. Geogr. 17, 95–105.
Corbel, J., 1959a. Erosion en terrain calcaire. Ann. Geogr.
73,
´
97–120.
Corbel, J., 1959b. Vitesse d’erosion.
Z. Geomorphol. 3, 1–28.
´
Cotton, C.A., 1917. Block mountains in New Zealand. Am. J. Sci.
44 Ž4., 249–293.
Cotton, C.A., 1948. Landscape. Whitcombe and Tombs,
Christchurch, 301 pp.
Crickmay, C.H., 1932. The significance of the physiography of
the Cypress Hills. Can. Field Nat. 46, 185–186.
Crickmay, C.H., 1974. The Work of the River. A Critical Study of
Central Aspects of Geomorphogeny. Macmillan, London, 271
pp.
Crickmay, C.H., 1976. The hypothesis of unequal activity. In:
Melhorn, W.N., Flemal, R.C. ŽEds.., Theories of Landscape
Development. SUNY, Binghamton, pp. 103–109.
Cunningham, F.F., 1969. The Crow Tors, Laramie Mountains,
Wyoming, USA. Z. Geomorphol. 13, 56–74.
Cunningham, F.F., 1971. The Silent City of Rocks, a bornhardt
landscape in the Cotterell Range, south Idaho. Z. Geomorphol.
15, 404–429.
Cuvier, G., Brogniart, A., 1822. Description Geologique
des
´
Environs de Paris. Doufour et d’Ocagne, Paris, 428 pp.
Dale, T.N., 1923. The commercial granites of New England. U. S.
Geol. Surv. Bull. 738, 488 pp.
de la Beche, H.T., 1839. Report on the Geology of Cornwall,
Devon and West Somerset ŽGeological Survey of England and
Wales.. Longman, Orme, Brown, Green and Longmans, London, 648 pp.
de la Beche, H.T., 1853. The Geological Observer. Longmans,
Brown, Green and Longmans, London, 846 pp.
Dixey, F., 1942. Erosion cycles in central and southern Africa.
Trans. Geol. Soc. S. Afr. 45, 151–167.
Drogue, C., Bidaux, P., 1992. Structural and hydrogeological
origin of tower karst in southern China ŽLijiang Plain in the
Guilin region.. Z. Geomorphol. 36, 25–36.
Dunn, E.J., 1908. The Buffalo Mountains. Mem. Geol. Surv. Vic.
6, 11 pp.
du Toit, A.L., 1937. Geology of South Africa. Oliver and Boyd,
Edinburgh, 611 pp.
Eyre, E.J., 1845. Journals of Expeditions of Discovery into Central Australia and Overland from Adelaide to King George’s
Sound, in the Years 1840—41. 2 volumes, Boone, London.
Fair, T.J.D., 1947. Slope form and development in the interior of
Natal, South Africa. Trans. Proc. Geol. Soc. S. Afr. 50,
105–118.
Fairbridge, R.W., Finkl, C.W., 1978. Geomorphic analysis of the
rifted cratonic margins of Western Australia. Z. Geomorphol.
22, 369–389.
Fairbridge, R.W., Finkl, C.W., 1980. Cratonic erosional unconformities and peneplains. J. Geol. 88, 69–86.
Falconer, J.D., 1911. The Geology and Geography of Northern
Nigeria. Macmillan, London, 295 pp.
Finkl, C.W., Churchward, H.M., 1973. The etched surfaces of
southwestern Australia. J. Geol. Soc. Aust. 20, 295–307.
Fogelberg, P. ŽEd.., 1985. Preglacial Weathering and Planation.
Fennia 163, 283–383.
Gauthier, R.C., 1980. Decomposed granite, Big Bald Mountain
area, New Brunswick. Current Research. Part B. Geol. Surv.
Can. Pap. 80-1B, 277–282.
Gilewska, S., 1964. Fossil karst in Poland. Erdkunde 18, 124–135.
Grant, D.R., 1989. Quaternary geology of the Atlantic Appalachian region of Canada. In: Fulton, R.J. ŽEd.., Geology of
Canada No. 1. Quaternary Geology of Canada and Greenland.
C.R. Twidaler Earth-Science ReÕiews 57 (2002) 37–74
Geological Survey of Canada. ŽVolume K-1 The Geology of
North America, Geological Society of America, Boulder, CO..
Canadian Government Publishing Centre, Ottawa, pp. 340–
393.
Hagedorn, H., 1980. Geological and geomorphological observations on the northern slope of the Tibesti Mountains, central
Sahara. In: Salem, M.J., Busrewil, M.T. ŽEds.., The Geology
of Libya, vol. 3. Academic Press, London, pp. 823–835.
Handley, J.R.F., 1952. The geomorphology of the Nzega area of
Tanganyika with special reference to the formation of granite
tors. C. R. Congr. Geol.
´ Int. ŽAlgiers. 21, 201–210.
Harris, W.K., Twidale, C.R., 1991. Revised age for Ayers Rock
and the Olgas. Trans. R. Soc. S. Aust. 115, 109.
Hassenfratz, J.H., 1791. Sur l’arrangement de plusieurs gros blocs
de differentes
pierres que l’on observe dans les montagnes.
´
Ann. Chim. 11, 95–107.
Herrmann, L.A., 1954. Geology of the Stone Mountain-Lithonia
district, Georgia. Ga. Geol. Surv. Bull. 61, 139 pp.
Hills, E.S., 1962. Geomorphology. Symposium on Geochronology
and Land Surfaces in Relation to Soils in Australia. Australian
Academy of SciencerCSIRO Division of Soils, Adelaide, pp.
10–13.
Ho, C.S., 1993. Seismic refraction survey at the proposed Kuala
Lumpur International Airport site, Sepang, Selangor. Proc.
24th Geol. Conf. Tech. Pap. 5, Geological SurveyrPrimary
Industry, Kuala Lumpur, Malaysia, pp. 285–295.
Holmes, A., 1918. The PreCambrian and associated rocks of the
District of Mozambique. Q. J. Geol. Soc. London 74, 31–
97.
Holmes, A., 1965. Principles of Physical Geology. Nelson, London, 1288 pp.
Holmes, A., Wray, D.A., 1912. Outlines of the geology of
Mozambique. Geol. Mag. 9, 412–417.
Horton, R.E., 1945. Erosional development of streams and their
drainage basins. Geol. Soc. Am. Bull. 56, 275–370.
Hovermann,
J., 1978. Untersuchungen und Darlegungen zum
¨
Inselbergproblem in der deutschen Literatur der 1. Halfte
des
¨
20 Jahrhunderts. Z. Geomorphol., Suppl. 31, 64–78.
Hutton, J., 1795. The Theory of the Earth. 2 volumes, Creech,
Edinburgh.
Hutton, J.T., Lindsay, D.S., Twidale, C.R., 1977. The weathering
of norite at Black Hill, South Australia. J. Geol. Soc. Aust. 24,
37–50.
Ingham, F.T., Bradford, E.F., 1960. The geology and mineral
resources of the Kinta valley, Perak. Fed. Malaya, Geol. Surv.
Dist. Mem. 9, 347 pp.
Jameson, R., 1820. Mineralogy. In: Brewster, D. ŽEd.., Edinburgh
Encycl. vol. 14. Blackwood, Edinburgh ŽII., chaps. 3–5 incl.
Jennings, J.N., 1963. Some geomorphological problems of the
Nullarbor Plain. Trans. R. Soc. S. Aust. 87, 41–62.
Jennings, J.N., 1976. A test of the importance of cliff-foot caves
in tower karst development. Z. Geomorphol., Suppl. 26, 92–97.
Jennings, J.N., 1985. Karst Geomorphology. Blackwell, Oxford,
293 pp.
Jessen, O., 1936. Reisen und Forschungen in Angola. Reimer,
Berlin, 397 pp.
71
Jones, T.R., 1859. Notes on some granite tors. Geologist 2,
301–312.
Jones, C.R., 1978. The geology and mineral resources of Perlis,
North Kedah and the Langkawi Island. Geol. Surv. Malays.,
Dist. Mem. 17, 257 pp.
Jones, D.C.K., 1999. Evolving models of the Tertiary evolutionary
geomorphology of southern England, with special reference to
the Chalklands. In: Smith, B.J., Whalley, W.B., Warke, P.A.
ŽEds.., Uplift, Erosion and Stability: Perspectives on LongTerm Landscape Development. Geol. Soc. London, Spec.
Publ. 162, pp. 1–23.
Judson, S., Ritter, D.F., 1964. Rates of regional denudation in the
United States. J. Geophys. Res. 69, 3395–3401.
Jutson, J.T., 1914. An outline of the physiographical geology
Žphysiography. of Western Australia. Geol. Surv. W. Aust.,
Bull. 61, 249 pp.
Jutson, J.T., 1934. The physiography Žgeomorphology. of Western
Australia. Geol. Surv. W. Aust., Bull. 95, 366 pp.
King, L.C., 1942. South African Scenery. Oliver and Boyd,
Edinburgh, 308 pp.
King, L.C., 1949. A theory of bornhardts. Geogr. J. 112, 83–87.
King, L.C., 1950. The study of the world’s plainlands. Q. J. Geol.
Soc. London 106, 101–131.
King, L.C., 1962. Morphology of the Earth. Oliver and Boyd,
Edinburgh, 699 pp.
Kingsmill, T.W., 1862. Notes on the geology of the east coast of
China. J. R. Geol. Soc. Ireland 10, 1–6.
Knopf, E.B., 1924. Correlation of residual erosion surfaces in the
eastern Appalachians. Geol. Soc. Am. Bull. 35, 633–668.
Kozlovsky, Y.A. ŽEd.., 1987. The Superdeep Well of the Kola
Peninsula. Springer, Berlin, 558 pp.
Lamego, A.R., 1938. Escarpas do Rio de Janeiro. Departamento
Nacional da Produçao Mineral ŽBrasil.. Serv. Geol. Min. Bol.
93, 71 pp.
Lane, E.W., Brown, C., Gibson, G.C., Howard, C.S., Krumbein,
W.C., Matthes, G.H., Rubey, W.W., Trowbridge, A.C., Straub,
L.G., 1947. Report of the subcommittee on sedimentary terminology. Trans. Am. Geophys. Union, pp. 936–938.
Larsen, E.S., 1948. Batholith and associated rocks of Corona,
Elsinore and San Luis Rey quadrangles, southern California.
Geol. Soc. Am. Mem. 29, 113–119.
Le Conte, J.N., 1873. On some of the ancient glaciers of the
Sierras. Am. J. Sci. Arts 5, 325–342.
Lehmann, H., 1954. Der tropische Kegelkarst auf den Grossen
Antillen. Erdkunde 8, 130–139.
Lewis, W.V., 1955. Remarks on Linton’s ‘The problem of tors’.
Geogr. J. 121, 483–484.
Linton, D.L., 1952. The significance of tors in glaciated lands. Int.
Geogr. Union Proc., 8th Gen. Assem. 17th Int. Congr., pp.
354–357.
Linton, D.L., 1955. The problem of tors. Geogr. J. 121, 470–487.
Linton, D.L., 1964. The origin of the Pennine tors—an essay in
analysis. Z. Geomorphol. 8, 5–24.
Lister, L.A., 1976. The erosion surfaces of Rhodesia. Unpubl.
PhD thesis, University of Rhodesia, Harare, Zimbabwe, 218
pp.
72
C.R. Twidaler Earth-Science ReÕiews 57 (2002) 37–74
Lister, L.A., 1987. The erosion surfaces of Zimbabwe. Zimbabwe
Geol. Surv., Bull. 90, 163 pp.
Logan, J.R., 1849. The rocks of Pulo Ubin. Verh. Genoots.
Kunsten Wetensch. ŽBatavia. 22, 3–43.
Logan, J.R., 1851. Notes on the geology of the straits of Singapore. Q. J. Geol. Soc. London 7, 310–344.
Lowry, D.C., 1970. The geology of the Western Australian part of
the Eucla Basin. Geol. Surv. W. Aust., Bull. 122, 201 pp.
Lowry, D.C., Jennings, J.N., 1974. The Nullarbor karst, Australia.
Z. Geomorphol. 18, 35–81.
Lyell, C., 1840. The origin of tubular cavities filled with gravel
and sand, called ‘sandpipes’, in the chalk near Norwich. Br.
Assoc. Adv. Sci. Rep. 9, 65–66.
Mabbutt, J.A., 1952. A study of granite relief from South West
Africa. Geol. Mag. 89, 87–96.
Mabbutt, J.A., 1961a. ‘Basal surface’ or ‘weathering front’. Proc.
Geol. Assoc. London 72, 357–358.
Mabbutt, J.A., 1961b. A stripped land surface in Western Australia. Trans. Inst. Br. Geogr. 29, 101–114.
Mabbutt, J.A., 1965. The weathered land surface in central Australia. Z. Geomorphol. 9, 82–114.
Mabbutt, J.A., 1966. The mantle-controlled planation of pediments. Am. J. Sci. 264, 78–91.
MacCulloch, J., 1814. On the granite tors of Cornwall. Trans.
Geol. Soc. 2, 66–78.
MacDonald, S., 1967. The geology and mineral resources of
North Kelantan and North Trengganu. Fed. Malaya, Geol.
Surv. Dist. Mem. 10, 202 pp.
MacDonald, R.C., 1976. Hillslope base depressions in the tower
karst topography of Belize. Z. Geomorphol., Suppl. 26, 98–
103.
McGee, W.J., 1897. Sheetflood erosion. Geol. Soc. Am. Bull. 8,
87–112.
Mennell, F.P., 1904. Some aspects of the Matopos: I. Geological
and physical features. Proc. Rhod. Sci. Assoc. 4, 72–76.
Miller, A.A., 1953. The Skin of the Earth. Methuen, London, 198
pp.
Milnes, A.R., Twidale, C.R., 1983. An overview of silicification
in Cainozoic landscapes of arid central and southern Australia.
Aust. J. Soil Res. 21, 387–410.
Molina Ballesteros, E., Campbell, E.M., Bourne, J.A., Twidale,
C.R., 1995. Character and interpretation of the regolith exposed at Point Drummond, west coast of Eyre Peninsula,
South Australia. Trans. R. Soc. S. Aust. 119, 83–88.
Mueller, J.E., Twidale, C.R., 1988. Geomorphic development of
City of Rocks, Grant County, New Mexico. N. Mex. Geol. 10,
73–79.
Mulcahy, M.J., 1973. Landforms and soils of southwestern Australia. Proc. R. Soc. W. Aust. 56, 16–22.
Oberlander, T.M., 1965. The Zagros Streams. Syracuse Geogr.
Ser. 1, 168 pp.
Oberlander, T.M., 1972. Morphogenesis of granite boulder slopes
in the Mojave Desert, California. J. Geol. 80, 1–20.
Obst, E., 1923. Das abflusslose Rumpschollenland in nordostlichen
¨
Deutsch-Ostafrica. 2 volumes, Mitt. Geogr. Ges., Hamburg,
35.
Ollier, C.D., 1960. The inselbergs of Uganda. Z. Geomorphol. 4,
43–52.
Ollier, C.D., 1965. Some features of granite weathering in Australia. Z. Geomorphol. 9, 285–304.
Palmer, J., Radley, J., 1961. Gritstone tors of the English Pennines. Z. Geomorphol. 5, 37–52.
Partridge, T.C., Maud, R.R., 1987. Geomorphic evolution of
southern Africa since the Mesozoic. S. Afr. J. Geol. 90,
179–208.
Passarge, S., 1895. Adamaua. Bericht uber die Expedition des
Deutschen Kamerun-Komitees in den Jahren 1893–94. Reimer,
Berlin, 573 pp.
Passarge, S., 1904. Rumpfflachen
und Inselberge. Z. Dtsch. Geol.
¨
Ges. 56, 193–215.
Paton, J.R., 1964. The origin of the limestone hills of Malaya. J.
Trop. Geogr. 18, 134–147.
Peel, R.F., 1966. The landscape in aridity. Trans. Inst. Br. Geogr.
38, 1–23.
Prestwich, J., 1855. The origin of the sand and gravel pipes on the
chalk of the London Tertiary district. Q. J. Geol. Soc. London
11, 64–84.
Pumpelly, R., 1879. The relation of secular rock-disintegration to
loess, glacial drift and rock basins. Am. J. Sci. Arts 17,
133–144.
Raeside, J.D., 1949. The origin of schist tors in central Otago. N.
Z. Geogr. 5, 72–76.
Richardson, J.A., 1947. Outline of geomorphological evolution of
British Malaya. Geol. Mag. 84, 129–144.
Richardson, J.A., 1950. The geology and mineral resources of the
neighbourhood of Chegar Perah and Merapoh, Pahang. Geol.
Surv. Malaya Mem. 4, 162 pp.
Roe, F.W., 1951. The geology and mineral resources of the
Fraser’s Hill area, Selangor Perak and Pahang, Federation of
Malaya, with an account of the mineral resources. Fed. Malaya,
Geol. Surv. Mem. 5, 138 pp.
Romanes, J., 1912. Geology of a part of Costa Rica. Q. J. Geol.
Soc. London 68, 133–136.
Ruxton, B.P., Berry, L.R., 1957. Weathering of granite and
associated erosional features in Hong Kong. Geol. Soc. Am.
Bull. 68, 1263–1282.
Sain Fond, B.F. de, 1784. Travels in England and Scotland in
1784. Žtransl. Geikie, A.. 2 volumes, Hopkins, Glasgow.
Salama, R.B., 1997. Geomorphology, geology and palaeohydrology of the broad alluvial valleys of the Salt River System,
Western Australia. Aust. J. Earth Sci. 44, 751–765.
Scrivenor, J.B., 1931. The Geology of Malaya. Macmillan, London, 217 pp.
Seager, W.R., 1981. Geology of the Organ Mountains and southern San Andres Mountains, New Mexico. N. Mex. Bur. Mines
Min. Res. Mem. 36, 97 pp.
Selby, M.J., 1977. Bornhardts of the Namib Desert. Z. Geomorphol. 21, 1–13.
Soderman,
G., 1985. Planation and weathering in eastern
¨
Fennoscandia. Fennia 163, 347–352.
Sweeting, M.M., 1950. Erosion cycles and limestone caverns in
the Ingleborough district. Geogr. J. 115, 63–78.
C.R. Twidaler Earth-Science ReÕiews 57 (2002) 37–74
Sweeting, M.M., 1958. The karstlands of Jamaica. Geogr. J. 124,
184–199.
Sweeting, M.M., 1972. Karst Landforms. Columbia Univ. Press,
New York, 362 pp.
Sweeting, M.M., 1990. The Guilin karst. Z. Geomorphol., Suppl.
77, 47–65.
Thomas, M.F., 1965. Some aspects of the geomorphology of
domes and tors in Nigeria. Z. Geomorphol. 9, 63–81.
Thomas, M.F., 1966. Some geomorphological implications of
deep weathering patterns in crystalline rocks in Nigeria. Trans.
Inst. Br. Geogr. 40, 173–191.
Thomas, M.F., 1989a. The role of etch processes in landform
development: I. Etching concepts and their applications. Z.
Geomorphol. 33, 129–142.
Thomas, M.F., 1989b. The role of etch processes in landform
development: II. Etching and the formation of relief. Z. Geomorphol. 33, 257–274.
Thomas, M.F., 1994. Geomorphology in the Tropics. Wiley,
Chichester, 460 pp.
Thomas, M.F., Thorp, M.B., 1985. Environmental change and
episodic etchplanation in the humid tropics of Sierra Leone:
the Koidu etchplain. In: Douglas, I., Spencer, T. ŽEds.., Environmental Change and Tropical Geomorphology. Allen and
Unwin, London, pp. 239–267.
Thornbury, W.D., 1954. Principles of Geomorphology. Wiley,
New York, 618 pp.
Thorp, M.B., 1969. Some aspects of the geomorphology of the
Air Mountains, southern Sahara. Trans. Inst. Br. Geogr. 47,
25–46.
Trendall, A.F., 1962. The formation of ‘apparent peneplains’ by a
process of combined lateritisation and surface wash. Z. Geomorphol. 6, 183–197.
Tricart, J., 1957. Mise au point: l’evolution
des versants. L’Inf.
´
Geogr. 21, 108–115.
Turner, F.J., 1952. Gefugerelief
illustrated by ‘schist tor’ topogra¨
phy in central Otago, New Zealand. Am. J. Sci. 250, 802–807.
Twidale, C.R., 1956a. Chronology of denudation in northwest
Queensland. Geol. Soc. Am. Bull. 67, 867–882.
Twidale, C.R., 1956b. Der ‘Bienenkorb’, eine neue morphologische Form aus Nord-Queensland, Nord-Australien. Erdkunde
10, 239–240.
Twidale, C.R., 1962. Steepened margins of inselbergs from northwestern Eyre Peninsula, South Australia. Z. Geomorphol. 6,
51–69.
Twidale, C.R., 1967. Origin of the piedmont angle as evidenced in
South Australia. J. Geol. 75, 93–411.
Twidale, C.R., 1972. The neglected third dimension. Z. Geomorphol. 6, 283–300.
Twidale, C.R., 1976. On the survival of palaeoforms. Am. J. Sci.
276, 77–95.
Twidale, C.R., 1978a. Early explanations of granite boulders. Rev.
Geomorphol. Dyn. 27, 133–142.
Twidale, C.R., 1978b. On the origin of Ayers Rock, central
Australia. Z. Geomorphol., Suppl. 31, 177–206.
Twidale, C.R., 1980. Origin of minor sandstone landforms. Erdkunde 34, 219–224.
Twidale, C.R., 1981. Granitic inselbergs; domed, block-strewn
and castellated. Geogr. J. 147, 54–71.
73
Twidale, C.R., 1982a. Granite Landforms. Elsevier, Amsterdam,
372 pp.
Twidale, C.R., 1982b. The evolution of bornhardts. Am. Sci. 70,
268–276.
Twidale, C.R., 1982c. Les inselbergs a` gradins et leur signification: l’exemple de l’Australie. Ann. Geogr.
91, 657–678.
´
Twidale, C.R., 1986. Granite landform evolution: factors and
implications. Geol. Rundsch. 75, 769–779.
Twidale, C.R., 1987a. A comparison of inselbergs developed in
various massive rocks. Ilmu Alam 16, 23–49.
Twidale, C.R., 1987b. Etch and intracutaneous landforms and
their implications. Aust. J. Earth Sci. 34, 367–386.
Twidale, C.R., 1988. Granite landscapes. In: Moon, B.P., Dardis,
G.F. ŽEds.., The Geomorphology of Southern Africa. Southern
Book Publishers, Johannesburg, pp. 198–230.
Twidale, C.R., 1990a. The origin and implications of some erosional landforms. J. Geol. 98, 343–364.
Twidale, C.R., 1990b. La corrosion chimique et sa signification
dans l’evolution
des paysages. La Terre et les Hommes.
´
Melanges
offerts a` Max Derruau. Association des Publications
´
de la Faculte´ des Lettres et Sciences Humaines de ClermontFerrand, France, pp. 317–334.
Twidale, C.R., 1991. A model of landscape development involving increased and increasing relief amplitude. Z. Geomorphol.
35, 85–109.
Twidale, C.R., 1994. Gondwanan ŽLate Jurassic and Cretaceous.
palaeosurfaces of the Australian Craton. Palaeogeogr., Palaeoclimatol., Palaeoecol. 112, 157–186.
Twidale, C.R., 1999. Oldlands and the Australian Craton. Phys.
Geogr. 20, 273–304.
Twidale, C.R., 2000. Early Mesozoic ŽTriassic. landscapes in
Australia; evidence, argument and implications. J. Geol. 108,
537–552.
Twidale, C.R., Bourne, J.A., 1975a. The subsurface initiation of
some minor granite landforms. J. Geol. Soc. Aust. 22, 477–
484.
Twidale, C.R., Bourne, J.A., 1975b. Episodic exposure of inselbergs. Geol. Soc. Am. Bull. 86, 1473–1481.
Twidale, C.R., Bourne, J.A., 1976. Origin and significance of
pitting on granite rocks. Z. Geomorphol. 20, 405–416.
Twidale, C.R., Bourne, J.A., 1978. Bornhardts developed in sedimentary rocks, central Australia. S. Afr. Geogr. 5, 35–51.
Twidale, C.R., Bourne, J.A., 1996. Development of the land
surface. In: Davies, M., Twidale, C.R., Tyler, M.J. ŽEds..,
Natural History of the Flinders Ranges. Royal Society of
South Australia, Adelaide, pp. 46–62.
Twidale, C.R., Bourne, J.A., 1998. Origin and age of bornhardts,
southwest Western Australia. Aust. J. Earth Sci. 45, 903–914.
Twidale, C.R., Campbell, E.M., 1984. Murphy Haystacks, Eyre
Peninsula, South Australia. Trans. R. Soc. S. Aust. 108,
175–183.
Twidale, C.R., Campbell, E.M., 1998. Development of a basin,
doughnut and font assemblage on a sandstone coast, western
Eyre Peninsula, South Australia. J. Coastal Res. 14, 1385–
1394.
Twidale, C.R., Centeno, J.D., 1993. Landform development at the
Ciudad Encantada, near Cuenca, Spain. Cuad. Lab. Xeol. Laxe
18, 257–269.
74
C.R. Twidaler Earth-Science ReÕiews 57 (2002) 37–74
Twidale, C.R., Vidal Romani, J.R., 1994. On the multistage
development of etch forms. Geomorphology 11, 157–186.
Twidale, C.R., Vidal Romani, J.R., Campbell, E.M., Centeno,
J.D., 1996. Sheet fractures: response to erosional offloading or
to tectonic stress? Z. Geomorphol., Suppl. 106, 1–24.
Urbani, F., 1986. Notas sobre el origen de las cavidades en rocas
cuarcıferas
precambricas
de Grupo Roraima, Venezuela. Inter´
´
ciencia 11, 298–300.
Van den Broek, E., 1881. Sur les phenomenes
d’alteration
des
´
`
´
depots
ˆ superficiels par infiltrations des eaux meteoriques.
´
Mem. Couron. Acad. R. Sci. Lett. Beaux-Arts Belg. 44.
Verstappen, H., 1960. Some observations on karst development in
the Malay archipelago. J. Trop. Geogr. 14, 1–10.
Vidal Romani, J.R., Twidale, C.R., 1998. Formas y Paisajes
Granıticos.
Ser. Monogr., vol. 55. Universidade da Coruna
´
˜
Servicio de Publicacions,
´ A Coruna,
˜ 411 pp.
Wahrhaftig, C., 1965. Stepped topography of the southern Sierra
Nevada, California. Geol. Soc. Am. Bull. 60, 781–806.
Walther, J., 1915. Laterit in West-Australien. Z. Dtsch. Geol. Ges.
67, 113–132.
Waterhouse, J.D., Commander, D.P., Prangley, C., Backhouse, J.,
1995. Newly recognised Eocene sediments in the Beaufort
River palaeochannel. Geol. Surv. W. Aust., 1993–94 Annu.
Rep. 82–86.
Wayland, E.J., 1921. Report on the geology of part of Karamoja,
by the Government Geologist. Geol. Surv. Protect. Uganda
Annu. Rep. 1920, 35–40.
Wayland, E.J., 1934. Peneplains and some erosional landforms.
Geol. Surv. Uganda Annu. Rep. Žfor year ending 31st March
1934. Bull. 1, 77–79.
Whitlow, J.R., 1978–1979. Bornhardt terrain on granitic rocks in
Zimbabwe. Zambia Geogr. Assoc. J. 33–34, 75–93.
Wilford, G.E., Wall, J.R.D., 1965. Karst topography in Sarawak.
J. Trop. Geogr. 21, 44–70.
Williams, G., 1936. The geomorphology of Stewart Island, New
Zealand. Geogr. J. 87, 238–337.
Willis, B., 1934. Inselbergs. Am. Assoc. Geogr. Ann. 24, 123–129.
Willis, B., 1936. East African plateaux and rift valleys. Stud.
Comp. Seismol. ŽCarnegie Institute Washington, DC. Publ.
470, 358 pp.
Wilson, A.W.G., 1903. The Laurentian Peneplain. J. Geol. 11,
615–669.
Woolnough, W.G., 1918. The physiographic significance of laterite in Western Australia. Geol. Mag. 54, 385–393.
Wopfner, H., 1969. Mesozoic era. In: Parkin, L.W. ŽEd.., Handbook of South Australian Geology. Geological Survey of
South Australia, Adelaide, pp. 133–171.
AW.T.B, 1896. Bukit Kitu. Selangor J. 4, 304–306.
Young, R.W., 1986. Tower karst in sandstone: Bungle Bungle
massif, northwestern Australia. Z. Geomorphol. 30, 189–202.
Young, R.W., Young, A., 1992. Sandstone Landforms. Springer,
Berlin, 163 pp.
Zwittkovits, F., 1966. Klimabedingte Karstformen in den Alpen,
¨
den Dinariden und in Taurus. Osterr.
Geogr. Ges. 108, 72–97.