Geomorphology 12 (1995) 151-165
Inversion of relief -
a component of landscape evolution
C.F. Pain a, C.D. Ollier b
aMinerals and Land Use Program Australian Geological Survey Organisation, PO Box 378, Canberra, ACT2601, Australia
b Centre for Resource and Environmental
Studies, Australian National University, Canberra, ACT0200,
Australia
Received 26 March 1994; revised 28 October 1994; accepted 15 November 1994
Abstract
Inversion of relief occurs when materials on valley floors are, or become, more resistant to erosion than the adjacent valley
slopes. As erosion proceeds, the valley floor becomes a ridge bounded by newly formed valleys on each side. Areas of lava
flows contain many examples of inversion of relief, but it is also common in areas of duricrusts. Inversion of relief is so
widespread in some areas that it should be regarded as a general component in a model of landscape evolution.
Inversion of relief has some important implications. Drainage lines will shift significantly over time. Slope-soil relationships
(catenas) have to be reassessed, because the regolith on the upper part of a hill slope may have developed under very different
conditions from those existing at present. The resulting catena is not a simple expression of soils and response to landscape
position. This also has important implications for geochemistry. Where present day ridge tops were once valley floors, geochemical signatures will reflect lateral water movement in the old landscape rather than simple in situ weathering and vertical
redistribution in the present landscape.
Finally, inversion of relief can produce erosion surfaces of very low relief that cannot be termed penepluirzs, pedipluins or
etchplains because they have a very different genesis. This underlines the importance of determining the complexities of landscape.
evolution before such genetic terms are applied to any landscape.
1. Introduction
“A river is in a hole from which it cannot escape.”
(Brunsden, 1993, p. 24). This paper presents a series
of examples which show that, in many landscapes, rivers do escape from “holes” by inversion of relief.
Inversion of relief refers to an episode in landscape
evolution when a former valley bottom becomes a
ridge, bounded by newly formed valleys on each side.
Inversion of relief is well-known in the context of lava
flows. It also occurs in landscapes with duricrusts. With
a few exceptions, however, inversion of relief is generally regarded as an oddity of landscape formation.
Summerfield ( 1991), for example, briefly discusses
the idea of inversion of relief, but only in a chapter on
duricrusts and not in his discussion of landform evoSSDIO169-555X(94)00084-0
lution. Tricart (1972) considers inversion of relief to
be a characteristic of tropical landscapes where cuirasses (ferricretes) are present, a view which is supported by Budel ( 1982). Twidale ( 1983) also
suggested that silcretes cause inversion of relief on a
major scale in parts of Australia. Inversion of relief is
so widespread that it deserves to be included in a general model for landscape evolution wherever materials
in valley bottoms are, or become, more resistant to
erosion than the adjacent valley slopes.
2. Examples of inversion of relief
These examples of landscapes indicate where inversion of relief is an important, if not the dominant, factor
in landscape development. This section of examples is
necessary for our discussion of some of the imphcations
of inversion of relief for landscape development and
interpretation.
Very clear examples of inversion of relief’ are found
in volcanic landscapes. particularly those dominated
by basalt, where lava flows down a valley. often covering alluvium, Oilier ( 1957, 1988 f provides many
examples. When drainage is re-established, the water
that once flowed down the axis of the valley usuahy
flows down the sides of the lava, as twin lateral streams.
5km
f
Fig. I Inversion of relief after a lava ffow. I A
1The
pre-volcanic iand-
Fig. 2. The Bullengarook lava tfow. west of Melbourne, Victoria. Austra-
scape, with alluvium in a valley floor. (B) Lava Rown down the valley.
lia. The fiat-topped lava flow is a long ridge, bounded by the Lerderberg
covering the alluvium. ( C ) Erosion formed twin lateral streams. leaving
River and Pyrite Creek, which have cut down about 60 m below the top
the pre-lava alluvium on a ridge. in inversion of relief’.
of the flow. Tbe age of the lava flow is about 3.5 Ma.
C.F. Pain, C.D. Ollier/Geomorphology
New valleys are eroded on each side of the flow. The
lava that was once in a valley bottom becomes a ridge
crest between two valleys (Fig. 1). Some lava flows
are still virtually intact between twin lateral streams,
but others have been dissected into isolated outliers.
This sequence of events is well documented by Hills
( 1960). The gravels beneath the flow may be valuable
sources of ground water, or may contain economic
deposits such as tin, gold or sapphires, in which case
they are known as deep leads. Because of the economic
value, deep leads have been traced in detail and in some
cases dated in many parts of the world, including California and Victoria (Australia). The nature and timing
of landscape evolution in such well-mapped areas is
beyond question.
Lava flow remnants and deep leads help to trace the
old, pre-volcanic drainage lines which may have been
partly destroyed by later erosion. Some examples have
very elegant simplicity (Fig. 2). Others have a more
complex history, with some post-basalt
drainage
responding as a simple lateral stream, but others
0
12 (1995) 151-165
153
crossing the lava flow to some extent, giving complex
patterns (Fig. 3).
Some old flows are in inverted relief, and have been
reduced to individual hills instead of long ridges. At
the extreme, only isolated, basalt capped hills occur
that are no longer associated with any obvious source.
Associated alluvium (the deep lead) shows that inversion of relief has occurred, but details of the paleodrainage can no longer be reconstructed with accuracy.
2.2. Inversion of relief associated with duricrusts
Various duricrusts can also give rise to inversion of
relief. Summerfield ( 199 1) states the general case and
provides a useful diagram (Fig. 4). The parallel
between inverted relief forms caused by lava flows and
those caused by duricrusts is often quite striking. Some
major differences, however, also occur, particularly in
the relationships between inverted relief and present
drainage lines, and the succession of events leading to
the inversion of relief.
5km
Fig. 3. Drainage modifications of the Campaspe. and Coliban Rivers, Victoria, Australia. (A) Original drainage as indicated by sub-basaltic
alluvium. (B) Maximum extent of lava, which flowed from the north. (C) Present-day drainage and lava distribution.
154
C.F. Pain. CL). Ollier/Gromorphology
Fig. 4. Stages in the development of inversion of relief in areas with
duricrust (after Summerfield. 199 I ) (A ) A duricrust forms in low,
points in the landscape. (B ) Subsequent erosion lowers the surrounding less resistant parts of the landscape. (C ) Eventually duricrustcapped residual hills are all that is left of the original duricrust that
was formed in low lying parts of the landscape.
Silcrete
Silcrete is a hard, grey rock made by silicification of
pre-existing deposits, especially quartz-rich alluvium.
Some of it has a cover of basalt from former lava flows,
but some has formed in alluvium far from any volcanic
activity. A good example is the upper Shoalhaven valley, Australia, where silicified alluvium is now perched
higher than the present river, by inversion of relief
(Ollier, 1991a).
An excellent example is provided by Barnes and Pitt
( 1976). The Mirackina Conglomerate, an alluvial silCrete in South Australia, caps elongated mesas which
are all that remains of a major drainage channel, complete with tributaries, that is more than 200 km long
(Fig. 5). Twidale ( 1983, 1984) also attributed extensive elongate plateaus capped with alluvial silcrete to
inversion of relief on a major scale.
12 (1995) 151-165
Other examples come from Cape York Peninsula,
where several stages in the inversion of relief process
can be detected. An early stage consists of broad linear
plateaus, bounded by erosion scarps, with active stream
channels flowing along the length (Fig. 6A). These
plateaus are up to 40 m above the surrounding erosional
plain. In some cases the channel degenerates to a chain
of ponds at its upstream end. At an intermediate stage,
narrow ridges, some capped with silcrete in alluvial
gravels, range in height from 10 to more than 60 m
above the surrounding erosional plains (Fig. 6B). The
final stage is represented by remnants with all alluvium
removed, leaving weathered bedrock silicified to silCrete (Fig. 6C).
Van der Graaff et al. ( 1977) note the presence of
silcrete in the higher parts of the West Australian landscape. The silcrete they describe formed in Tertiary
fluviatile gravels and sands which are now preserved
as remnants on interfluves.
Inversion of relief can result from silica-cemented
deposits of considerable antiquity. The Permian Kirup
Conglomerate (Western Australia), is a linear silicified
alluvial unit up to 2.5 km wide and more than 80 km
long, now resting on upland divides and, therefore,
indicating inversion of relief (Fairbridge and Finkl,
1978).
Ferricrete
Ferricrete, a hard surficial material rich in iron oxides
and hydroxides, is sometimes called “laterite”. That
term is, however, potentially misleading because the
definition and mode of formation of “laterite” are controversial.
Many writers have pointed out that iron tends to be
mobilised on upper slopes and precipitated on lower
slopes (Folster. 1964; Maignien, 1959). In the Kalgoorlie region of Western Australia, ferricrete is especially common in old gravels along valleys, or at least
the edges of valleys. Other examples of valley floor
ferricrete are found in North Queensland, Australia
(Pain and Ollier, 1992). In Kalgoorlie and North
Queensland, the landscape contains many small plateaus capped by ferricrete, often with associated alluvium. But if ferricrete is formed in valley bottoms, why
is it found on plateau tops? The answer seems to lie in
inversion of relief (Ollier, 1991 b) .
An obvious example of inversion of relief with ferricrete is provided by the nodular iron ore of the Robe
CF. Pain, CD. Ollier/Geomorphology
River deposit, Western Australia. The iron ore extends
for tens of kilometres along the courses of old valleys,
and the modem valley follows as a lateral stream
(MacLeod, 1966; Twidale et al., 1985; Hall and Kneeshaw, 1990) (Fig. 7). It is not clear to what extent the
iron ore was transported as nodules, and how much was
formed in situ by chemical precipitation. However, its
similarity to the Mirackina Conglomerate (Fig. 5) is
striking.
Ollier and Galloway ( 1990) showed that inversion
of relief in areas with ferricrete is found in Australia,
parts of Africa, India and elsewhere, and may be a
general feature of geomorphology in such areas.
Calcrete
Many examples of calcareous-cemented
alluvium
leading to inversion of relief are known (see, for example, Maizels, 1987, and references therein). In semiarid
regions calcium carbonate may be deposited along
.-
Present drainage
--_)
Paleo-flow directions
I
155
stream lines. When this is hardened into a calcrete, it
may act like a lava flow, becoming less erodable than
the neighbouring valley sides. New streams then form
lateral to the calcrete, and eventually cause inversion
of relief.
Miller (1937) described an example from Saudi
Arabia as “drainage lines in bas-relief’
(Fig. 8). He
attributed this inversion of relief, or “suspended drainage’ ’ , to calcareous cementing of materials along drainage lines and subsequent erosion of the less resistant
materials from between the channels. It is widespread,
occurring over an area of 10,000 km2 in eastern Saudi
Arabia.
Another example from the Middle East was observed
by Brown (1960), who described “gravel trains”
lying 30-60 m above the present river beds. He noted
that a “dendritic drainage pattern is etched out in bas
relief, a caliche [calcrete] carapace having protected
the Pleistocene channels while the wind removed the
a-0
j
0
12 (1995) 151-165
50 km
I
Fig. 5. The Mirackina Conglomerate (after Barnes and Pitt, 1976). The paleo-drainage line is clearly outlined by silcrete-capped mesas, an
excellent example of inverted relief.
156
C.F. Pain, C.D. Ollier/Geomorpholog.v
12 (1995) 151-165
/ 3
C
Fig. 6. Stages of inversion of relief caused by siliceous cementing in Cape York Peninsula. (A) Active drainage on a linear plateau, with a
siliceous hardpan formed in both alluvium and saprolite. (B) Ridges with silcrete. (C) Isolated remnants of old valley floors.
C.F. Pain, C.D. Ollier/Geonwtphology
2,
1
n
”
I
Femkrete-capped mesas
157
12 (1995) 151-165
Y-__/-+
__,
wm
/
Fig. 7. Distribution of the ferricrete-capped mesas that define the Robe River paleoriver in Western Australia (after MacLeod,
the modem channels are often subparallel to paleochannels as lateral streams.
inter-stream divides’ ’ . Other examples are reported
from Oman (Maizels, 1990; Beydoun, 1980).
In west Texas and New Mexico, in the United States,
Reeves ( 1983) described long ridges, capped with cal-
1966). Note that
Crete, with internal drainage depressions. He explained
them as resulting from “relief inversion of calcium
carbonate-cemented drainage channels.” (Reeves
1983, p. 180, see also his figs. 5 and 6, p. 182).
__----L-.
, Paleosufface
____------__-------_---------_____-----
Limestone
B
Shale
Fig. 8. Inversion of relief in calcrete over limestone and shale bedrock in Arabia (after Miller, 1937).
158
C. F. Pain. C.D. Ollirr/Gromorphulo~~
Other examples come from Western Australia,
where the potential economic uranium content has led
to extensive exploration (Butt et al.. 1977 ) Carlisle
( 1983) reports on valley calcretes near Yeelirrie in
Western Australia. These calcretes occur for tens of
kilometres along the central parts of broad drainage
depressions (see fig. 2 in Carlisle, 1983, p. 186 ). and
commonly stand l-2 m above the surrounding valley
floors. This appears to be an early stage of inversion of
relief.
Particularly good examples are found in the Ashburton Valley, Western Australia, where perched calcretes now capping mesas are the more resistant
remnants of ribbons of carbonate formed under previous drainage regimes (Mann and Horwitz. I979 ) Fine
grained alluvial clays and bands of rounded river gravels occur between the calcrete and bedrock. Fig. 3 in
Mann and Horwitz ( 1979) is a photograph of one such
mesa which now stands 30 m above the surrounding
plain; a clear example of inversion of relief.
Gypcrete
Gypcrete forms a duricrust which can be of geomorphic importance, but research on the topic appears to
be lacking (see, for example, Tucker, 1978 ). Watson
( 1983) noted that gypcrete plays an important role in
limiting aeolian deflation of gypsum-cemented
sands.
He also points out that the formation of ground water
gypsum crusts may lead to inversion of relief.
Gypcrete occurs overlying elastic sediments ranging
from clay to pebbles in the Lake Eyre Basin ( Wopfner
and Twidale, 1967). They consider the gypcrete to
have formed on low angle alluvial plains which have
subsequently been dissected, the gypcrete forming a
cap rock. This is not. strictly speaking, inversion of
relief in the sense used in this paper. but shows that in
some circumstances gypcrete would have the same
effect as other duricrusts described above.
Other types of inversion
Inversion of relief can possibly occur even though
induration is minimal. Bryan ( 1940). and later Mills
( 198 1, 1990) described inversion of relief in the Appalachians of the USA. Floors of valleys up to 1 km wide
become infilled by large ( > 1 m) boulders of quartzite
which armour the floors of channels and so prevent
further erosion of the floors by running water. Erosion
then becomes focused on the valley side slopes. These
12 (1995) 151-16.5
are progressively eroded until the old depression floor
is at a higher elevation than the original ridge. The result
is ridge tops with caps of bouldery colluvium, and
beneath them deeper saprolite than underlies the tops
of many uncapped ridges, as determined by seismic
methods.
3. Discussion
3.1. Indicators of irwersion oj’relief
In our research and mapping we have detected a
considerable reluctance to accept inversion of relief as
a reasonable hypothesis of landscape development.
Some situations described in the literature, that appear
to us to be clear examples of inversion of relief, are
explained by authors in different ways. How can the
doubters be convinced? We list here some criteria for
inversion of relief: the more that can be demonstrated.
the clearer the case becomes.
~ Concave elongate plateau surfaces. or lines of plateau remnants. formed initially as concave valley
bottoms.
-~ Alluvial channels now in higher parts of the landscape. They may be filled with alluvium, which may
be simply channel sand, or deep leads if enriched in
such materials as gold or tin.
- Accumulations of iron oxides, silica, calcium carbonate or similar materials on the plateau edges.
These materials commonly have an apparent dip
towards the centre of the plateau, and were originally
deposited on valley bottoms or lower slopes.
- Mesas or narrow plateaus which have a dendritic
pattern in plari. Such patterns are found in South
Australia (Fig. 5 ), and the Robe River area (Fig.
7). They have also been described from the Kalgoorlie region (Ollier et al.. 1988).
- Continuity of inverted relief with normal relief. This
can take two forms (Fig. 9 ) :
(a) The upper valley remains normal, but the lower
valley is inverted. An example is provided by the
Loddon Valley. in Victoria, Australia.
(b) The lower valley remains normal, but inversion is
occurring in the upper reaches. Fig. 10 provides an
example from northern Queensland, Australia.
C.F. Pain, C.D. Ollier/Geomorphology
12 (1995) 151-165
159
. Concave saprolite profile below paleo-valleys. One
of the best described examples of this is found in the
Lawlers area, Western Australia (Anand and Smith,
1993) (Fig. 11).
. The presence of alluvial gravel is the best indicator
of inversion of relief, and several published diagrammatic sections suggest that inversion of relief
has occurred, even though not described by the
authors.
Fig. 9. Continuity of inverted relief with normal relief. This can take
two forms: (A) The upper valley remains uninverted. but the lower
valley is inverted. (B) The lower valley remains uninverted, but
inversion is occurring in the upper reaches.
Fig. 10. An example of inversion of relief from northern Queensland.
Moving upstream, the active channel of Mistake Creek first becomes
inactive, and then passes into a chain of depressions. This in turn
passes to a narrow plateau, and then a series of small mesas. In their
lower reaches, Muddy Water, Mistake and Toby Creeks are all active,
and part of the same “land surface”. However, Mistake Creek
becomes separated by a scarp from the other two in its upper reaches,
and if only this part of its catchment were studied, it would appear
to be on an older land surface than the other two streams. In reality,
this anomalous situation is a result of inversion of relief.
At Lawlers, Western Australia, an old valley (Fig.
11) is filled with a large mass of poorly sorted sediment
but has a distinct channel at the base with alluvial sands
(Anand and Smith, 1993). Furthermore, iron appears
to have been precipitated at the boundary between the
saprolite and overlying sediments, at the permeability
contrast where precipitation is most expected, with
pisolitic fenicrete in the more porous upper material,
and vesicular ferricrete in the denser saprolite. Moreover, the saprolite is concave up, following the old
valley, rather than parallel to the present topography.
All this is very hard to explain by any mechanism other
than inversion of relief.
A further example comes from Burkina Faso in West
Africa. Fig. 12 shows a section based on one published
by Zeegers and Lecomte ( 1992). The concave ground
Fig. 11. Regolith-landforrn
relationships in the Lawlers area, Westem Australia (simplified from Anand and Smith, 1993). Note the
concave upwards form of the higher part of the landscape. Within
this saucer-shaped area mixed colluvium and alluvium overlies fine
grained transported material. Below the transported material the base
of the saprolite is also concave upwards. These features all suggest
inversion of relief. Note the occurrence of ferricrete in both transported and in situ material.
160
C.F. Pain. C.D. Ollier/Geomorpholog~
12 (1995) 151-165
Fig. 12. Cross section of a plateau in Burkina Faso, West Africa, simplified from Zeegers and Lecomte ( 1992). The concave mottled zone and
the distribution of ferricrete on each side of channel sand suggests that the plateau was originally a valley floor. The Analysis of arsenic suggests
that it was derived laterally from the left and not from underlying rock
surface of the plateau, the concave saprolite parallel to
the (presumed) old valley, and especially the presence
of channel sands, all suggest inversion of relief. The
geochemical data also suggest enrichment from the ore
vein on an old valley side by lateral movement of solutions. Yet Zeegers and Lecomte do not consider inversion of relief.
3.2. Implications
of inversion of relief
Catenas
The catena concept marks an important advance in
the appreciation of landform and regolith evolution.
The classic analysis of soil or weathering profiles.
although not overtly stating the case, treats the weathering profile like a test tube or column that developed
by vertical movement of elements, water, or particles.
In contrast, the catena concept takes in lateral movements such as lateral ground water flow, the sideways
transfer of solutions, and hillside creep. The catena
concept is important for the formation of footslope and
valley duricrusts, because these duricrusts are created
in large part from the lateral movement of the cementing agents.
After inversion of relief, the geometrical catena is
no longer a genetic catena in any simple sense. The
upper part of the landscape originally formed as a valley
bottom, probably under different conditions from those
of today, receiving materials from a higher part of a
landscape that has now disappeared. The lower slopes
are younger, and cut across pre-weathered materials.
The upper and lower parts of the catena have funda-
mentally different histories. The linkages between them
become more complex in the post-inversion environment.
An example can serve to illustrate the significance
of this point. Van de Graaff ( 1983) described a catena
in Western Australia with silcrete on the upper slopes
and various ferricrete profiles on lower slopes. His
description of the silcrete profiles and his depiction of
sections showing apparently sloping silcretes (as in his
fig. 1) could suggest that this is an example of inversion
of relief. If this is so, the silcretes were formed long
before the profiles on the lower slopes, and speculation
about the geochemical relations between silcrete and
ferricrete in this area are not warranted. We have not
visited this area and are not challenging Van de Graaff s
interpretation but are merely providing an example of
a situation that could be questioned and used as an
example of inversion of relief.
Thus, in areas of inversion of relief, it is important
to determine whether a “catena” is made up of a series
of soil or weathering profiles all formed under the same
environmental conditions except those related to slope,
or has a complex geomorphic history in which upper
parts of the catena are older and geochemically different from the lower slopes.
Hydrology
Inversion of relief also has implications for the interpretation of fluctuations in water tables. Duricrusts,
especially ferricrete, have been used as paleo-climate
indicators. Ferricrete (laterite) has been assumed to
form in a humid tropical, perhaps seasonal, climate,
C.F. Pain. C.D. Ollier/Geomorphology
and its presence in areas with different climates has
been used as evidence of a change in climate. This
change in climate has been linked to water table
changes, especially falling water tables (Butt, 1987).
If, however, the inversion of relief hypothesis is correct
for large areas where ferricrete now exists, water table
fluctuations in these areas would have varied widely in
different parts of the landscape during its evolution.
There would be changes from high water tables in valley bottoms to low water tables in the same areas as
they become the higher parts of the landscape. These
changes can take place without any regional climatic
changes.
Geochemical
landscapes and landscape lowering
We can think of the regolith in one, two or three
dimensions; that is, the soil or weathering profile, the
catena, and the landscape.
In the case of ferricretes an interesting situation
arises. We have suggested that iron in the landscape is
concentrated in the lower slopes and valleys as a crust.
At the same time other parts of the landscape are
depleted of iron, some becoming the mottled and pallid
zones. Studies of ferricretes with a vertical perspective
showed repeatedly that the amount of iron in the ferriCrete could not be derived from the underlying profile.
Various solutions were proposed, including blowing in
extra iron oxides (Du Bois and Jeffery, 1955), or general landscape lowering (Trendall 1962). Trendall
( 1962, p. 186), using the iron content of granite and
the laterite over it in Uganda, calculated that 14 m of
granite would be required to produce 1 m of laterite.
He amended this to 20: 1 to allow for some lateral movement of iron to account for the presence of laterite over
quartzite, and then proposed an overall process of
ground surface lowering to account for a surface covered with laterite.
Landscape lowering is still preferred by some workers (e.g. Tardy and Roquin, 1992). Lateral and oblique
movement of iron in solution, however, enables the
iron to be concentrated from a large area into a small
area, and the problem of iron supply disappears. With
inversion of relief we are dealing with absolute rather
than relative accumulation.
One old idea is that ferricrete (laterite) was associated with peneplains of great perfection (e.g. Woolnough, 1927). Jutson (1914) tried to account for the
landscape of much of the shield area of Western Aus-
12 (1995) 151-165
161
tralia by this hypothesis. The landscape, however, lacks
sufficient iron to make a thick sheet of ferricrete across
an entire landscape. If such a sheet existed in the past,
a great deal of iron has been lost without trace. The
hypothesis of inversion of relief requires only a small
part of the landscape to contain ferricrete at any particular time. The landscape geochemistry never has to be
extraordinary, as in the hypothesis proposed by Jutson
(1914).
Geochemical exploration
The hypothesis that at least some landscapes develop
by inversion of relief is not of mere academic interest,
but affects fundamental concepts of geological and
geochemical mineral exploration (see Ollier, 1994, for
a more detailed discussion). In particular, the concept
of genetic relationships between samples that are vertically disposed still dominates most thinking in mineral exploration. Analyses of down-bore samples and
vertical profile samples are routinely carried out as if
the relationships were proven. Conventional ways of
presenting analytical data in diagrams and weathering
indices, and diagrams that place samples on axes purporting to show genetic relationships are routine. Some,
such as Schellmann diagrams, are specifically related
to laterite formation. All assume that vertical relationships prevail.
If lateral flow is responsible for the movement of
cementing materials, such as iron and silica, the duricrust may have virtually no relationship to the material,
either bedrock or weathering profile, which underlies
it. Such appears to be the case illustrated in Fig. 12,
where anomalies relate much better to the source now
off the plateau than to the underlying rock.
Erosion surfaces
In extensive, low relief areas like Western Australia,
the small scarps and breakaways become very significant and eye catching. Scarps may be held up by quite
minor amounts of duricrust (so far as the total landscape is concerned), and this is not readily appreciated.
The presence of “laterite” profiles on high mesas and
buttes surrounded by plains cut on bedrock at the level
of the base of the laterite profile has been interpreted
as meaning that a former continuous deep weathering
layer has been stripped leaving only a few residuals
(e.g. Jutson, 1914; Finkl and Churchward, 1974). Too
easily an imaginary “peneplain” is cast across the
C.F. Pain. C.D. Ollier/Gromorphology
162
EMBLEY
RANGE
Present
I
Ferricrete
I2 (1995) 151-165
m
Alluvium
ground
m
surface
Mottled
zone
0
Saprolite
Bedrock
Fig. 13. Diagrammatic representation of regolitb distribution in the Embley Range-Wenlock
River area, Cape York Peninsula (from Pain,
1992). Deep weathering profiles and ferricrete are confined to present valley floors and old valley floors now in inverted relief. These relations
apply over much of the peninsula.
mesas and produces a phantom peneplain, completely
covered with duricrust. The same applies to the silcrete
exposures that occur in many parts of Australia.
Alternatively,
the deep weathering, silcrete, and
“laterite” may have been restricted to valley locations.
The evidence from Cape York Peninsula, for example.
suggests that this part of Australia never had a continuous deep weathering layer (Pain and Ollier, 1992).
Instead, deep weathering is largely confined to present
valley floors, and to inverted drainage lines (Fig. 13 )
Similarly, Butt ( 1985, p. 431) suggested that the
“prominence of silcrete might belie its true abundance
and it is not inconceivable that not only are silcrete
exposures mostly confined to elevated scarps but that
silcretes are uncommon in the surrounding plains”.
Erosion surfaces of great perfection (compared to
hilly or mountainous regions) exist, but some of the
flattest (e.g. the Yilgam block in Western Australia,
and Cape York Peninsula in North Queensland) reveal
complexities, including inversion of relief, that cannot
be ignored. These areas have been described as peneplains (Jutson, 1914), pedipluins (King, 1949) and
etchplains (Finkl, 1979; Thomas, 1989). None of these
terms can be strictly applied to these surfaces if our
hypothesis is true. Perhaps another name is required.
And if inversion of relief is as common as we suspect,
the resulting erosion surfaces may be as common as
any of those previously defined by other authors.
3.3. Drainage evolution
Drainage evolution resulting from inversion of relief
raises two main questions. First, and particularly in the
case of lava flows, why do rivers move out of the valleys
rather than remaining in them and eroding the lava
away? Hundreds of kilometres of known lateral streams
show that lava does displace drainage lines. The mechanism works, so the slight convexity of flows, lateral
barriers, and impermeability of the lava edge are apparently effective in locating new drainage along the edge
of the flow.
A second question is why the lateral stream does not
simply cut down vertically. Perhaps in some instances
it does, but in many cases simple down cutting and the
C.F. Pain, C.D. Ollier/Geomorphology
formation
of symmetrical
valleys would rapidly
destroy the narrower lava flows. The destruction seems
to be more common downstream, as in the Lodden
valley, where depth of dissection has been greater than
upstream, where the lava is still intact. It would seem
that lateral streams often have a tendency to move away
from the lava flow, into the bedrock. Perhaps this is
because the bedrock, at least in Australia, is often
highly weathered and, therefore, less resistant than
lava. Slope retreat is effectively held up by a hard barrier at the top of the slope (Ollier and Tuddenham
1962). If valley widening is controlled by parallel slope
retreat, the hard lava prevents retreat whereas the opposite, softer, more erodable side can retreat readily and
the valley bottom itself gradually moves laterally away
from the lava.
Duricrusts, being highly resistant to erosion, have
the same effect, although the lateral streams are often
further away from the former valley floor. The duricrust
effectively holds up slope retreat on the site of the valley
floor. But on the weathered valley sides, slope retreat
is not held up and the valley bottom migrates away
from the duricrust.
4. Conclusions
Inversion of relief dominates the story of landscape
evolution in many volcanic areas. For example, it
would not be possible to make sense of the landscapes
in the volcanic province of western Victoria, or even
the mountainous landscapes of eastern Victoria, without an appreciation of the inversion of relief mechanism.
Landscapes
with duricrust are also commonly
greatly influenced by inversion of relief. This is not
surprising because a great deal of induration can be
observed in the lower parts of landscapes. Iron, silica,
calcium carbonate and gypsum all cement valley floor
or lower valley side materials in many present landscapes. When such cementing occurs, these locations
become more resistant to erosion than the areas in
between, and in time become the higher parts of the
landscape.
In models of landscape evolution, modem textbooks
of geomorphology
expound various ideas, but none
suggest that inversion of relief is a major component
I2 (1995) 151465
163
landscape development. Given the widespread presence of lava flows and duricrusts in the world, inversion
of relief may be a dominant feature in landscape evolution.
Acknowledgements
C.F. Pain publishes with the permission of the Executive Director, Australian Geological Survey Organisation.
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