Shore platforms:
feature
by A. S.
a
neglected
coastal
Trenhaile
The sea’s
ability to produce intertidal rock platforms was first commented
than a century ago (Ramsey, 1846; Dana, 1849). The literature
dealing with shore platforms, however, is still not large and much of it is
not readily available in university libraries. The literature, furthermore,
generally refers to a few well studied areas, particularly south of Melbourne
and around Sydney in Australia, the Auckland environs and eastern
Northland in New Zealand, Oahu Hawaii, southern Britain and southern
Japan. This distribution, which reflects the activities of a rather small
number of workers, is undoubtedly only a minute sample of the platforms
which exist around the world. Until the early 1960s, the study of shore
platforms was by default the almost exclusive domain of Australasian
workers. This work, which still exerts a strong influence on the treatment of
shore platforms in modern texts, was descriptive and generally concerned
with determining whether platforms are the product of wave erosion or
weathering processes. Genetic criteria usually included the degree of shelter
provided by platform sites, and the elevation of the platforms in relation to
specific tidal levels. It is ironic that in Britain the neglect of contemporary
shore platforms occurred at a time when great importance was attached to
the identification and interpretation of their ’raised’ counterparts. The first
studies specifically concerned with British shore platforms are less than
twenty years old. Recent work has been particularly concerned with:
on more
a) investigating relationships between platform morphology and aspects of
morphogenic environments; and
b) identifying and measuring the rates of erosion of the processes operating
on shore platforms.
Evidence provided by this work has necessitated a re-evaluation of the
view that shore platforms are quasi-static inherited features from an earlier
period when sea level was similar to today’s.
their
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2
This discussion is motivated by the need for a review of past and present
in shore platform research, at a time when coastal
geomorphology is increasingly concerned with depositional features, and
with the processes of the beach and surf zones. The subject is an important
one, not only because shore platforms constitute a significant proportion of
many coasts, but also because the explanation of their form and
development is a vital precursor to the interpretation of their raised
developments
counterparts.
I
Processes of
platform development
The relative contributions of marine and subaerial processes to the
development of shore platforms has been a contentious issue for more than
a century. Early coastal studies in the vigorous storm wave environments of
the northern hemisphere emphasized mechanical wave erosion (De la
Beche, 1839; Ramsey, 1846; Davies, 1896; Fenneman, 1902), but in
Australasia and elsewhere around the Pacific fringe it was often accorded a
secondary role to subaerial weathering (Dana, 1849; Bell and Clarke,
1909). Modern work has generally continued to stress mechanical wave
erosion in the northern hemisphere, but several early workers did consider
that subaerial weathering facilitates effective wave erosion, or even that it is
a vital precursor to platform development (Jukes-Browne, 1893; Geikie,
1903). Although subaerial weathering generally continues to be
emphasized in swell wave environments, shore platforms have also been
attributed to wave erosion in these areas. Several workers have suggested
that both wave-cut and weathered platforms occur in the southern
hemisphere, as well as wave-cut ramps which have been subsequently
bevelled by subaerial processes.
The processes acting on shore platforms have usually been inferred from
their morphology, although this evidence may be ambiguous. A myriad of
processes operate on platforms (Hof~meister and Wentworth, 1940), but the
two suites, mechanical wave erosion and weathering, are generally
considered to be dominant.
-
1
Mechanical
wave
erosion
The processes of mechanical wave erosion include breaking wave shock,
water hammer, air compression in joints, hydrostatic pressure, cavitation,
and abrasion (Sanders, 1968a). The most effective processes are generally
the most restricted in extent. Water shock, hammer, and air compression,
which are probably the most significant erosional processes, only operate
efficiently within the surf-breaker zone. The quarrying of rock fragments by
air compression and water hammer, is probably the most effective process
in the storm wave environments of the northern hemisphere, but it is also of
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3
great importance in the swell
wave environments around the Pacific fringe.
Effective abrasion requires the presence of a suitable abrasive. Robinson
(1977a) found that erosion on the ramp at the back of the platform is fastest
in winter, when abrasion is most active. Abrasives are lacking on the
platform at lower level, however, and quarrying, by compression of air in
cracks produced by alternate wetting and drying, is the dominant process.
Abrasion may occasionally produce smooth, well-planed surfaces, but its
effect is often greatest along lines of weakness, since the back and fro
motion of abrasives appears to be more effective than direct frontal attack.
Potholes may develop at the foot of scarps, but they are rarely sufficiently
numerous to constitute a major element of platform downwearing, unless
the rock is particularly resistant to wave quarrying. Little is known about
the effect of submarine abrasion on platform development. Reffell (1978)
reported that abrasion operates at the base of low tide cliffs near Sydney,
although it is not significant in Tasmania (Sanders, 1968a).
Slow rates of erosion have generally prevented the use of wave tanks and
flumes in studying the development of hard rock coastlines. This problem
may be partly overcome by the use of substitute materials which are more
sensitive to wave attack than natural rock. Whether the morphological
response of these materials to erosion is similar to that of natural rock,
however, is questionable. Nevertheless, on two occasions wave flumes have
been used to subject plaster and cement blocks to wave attack. Sunamura
(1975) found that the type of wave reaching the cliff foot determines the
rate of .platform development, as well as its elevation. Sanders (1968b)
argues that since the notches produced in the blocks contained no
horizontal elements, there is no evidence to suggest that horizontal, high
tidal platforms are cut by storm waves. As Gill (1972a) has noted, however,
the results are inconclusive, since Sander’s experiments were of insufficient
duration to permit the development of horizontal surfaces in the blocks.
2
Weathering
Bartrum and Turner (1928) first described the lowering, smoothing, and
levelling of shore platforms by weathering processes. The efficacy of water
layer weathering is greatest in fine grained, permeable materials. Although
it has been suggested that these processes may initiate platforms in some
cases (Wentworth, 1938; Ongley, 1940; Tricart, 1959), most workers limit
their operation to the secondary modification of wave-cut platforms. Water
layer levelling operates wherever spray and splash accumulate, although it
may be most effective in a zone extending from high tide level to about
1/3 m above (Sanders, 1968a). Most workers agree that the processes are
associated with wetting and drying, salt crystallization, and the movement
of solutions through rock capillaries, but their precise mechanisms have not
been elucidated. Emery and Foster (1956) evoked partial weathering to
account for intertidal nips in tuff. They suggested, that iron, aluminium,
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4
ions, as well as calcium and magnesium, are removed by
hydration, facilitated by frequent wetting and drying. The
most significant process may be base exchange hydration, or the loosening
of grains by swelling (Emery, 1960). Bowman (1970) discussed the formation of weathered depressions, enclosed by raised rims of dark-stained,
indurated joint planes, near Sydney. They are best developed on
sedimentary rocks with a high proportion of non-silaceous material.
Physical processes which contribute to the breakdown of the rock cement
include the expansion and contraction of alternatively wet and dry clay
and titanium
solution and
minerals, and salt crystallization within the rock interstices. Chemical
processes appear to be ass6ciated with the weathering series carbonate and
siderite, or pyrite and iron sulphates, to limonite. Material in solution is
washed from the pits and deposited and oxidized in the joint planes.
Bowman explained the development of a sequence of forms, ranging from
shallow basins with raised rims above the upper level of wave abrasion, to
blocks with flat summits with grooves along the joint planes, in very
exposed locations at lower elevations. Sanders (1968a) also attributed the
development of blocks with flat centres to water layer levelling, their raised
rims being associated with narrow joints which are kept moist by capillary
movement of water. The formation of polygonal pits or honeycombs have
been attributed to similar processes (Bartrum, 1936; Hills, 1940; Jutson,
1954; Cotton, 1963).
Although honeycombs and other minor forms in southern England may
be attributed to salt weathering, several factors prevent it from assuming a
major role in platform development. Low rates of evaporation, semi-diurnal
tides, and sloping platforms inhibit water layer levelling in the storm wave
environments of the northern hemisphere. Probably of greatest significance,
however, is the efficiency of mechanical wave erosion, since rock surfaces
rarely survive long enough for slow weathering to become effective.
Solution may be active on any lithology which has a significant carbonate
content, or a carbonate cement. Emery (1946) considered that solution is
partly responsible for the formation of shallow basins with raised rims on
sandstone platforms. This suggests that similar forms, which have been
attributed to water layer levelling, may be partly solutional in origin.
Wentworth (1939) attributed the formation of limestone benches in Hawaii
to solution by fresh ground or rain water, since warm sea water is normally
saturated or supersaturated with bicarbonates. Solution by sea water,
however, is enhanced by water flow and turbulence in the surf zone (Kaye,
1957), as well as by bio-chemical processes.
Chemical weathering is effective in the southern hemisphere, but the role
of frost and ice is probably of greater significance in the storm wave
environments of the north. Rapid cliff recession and the formation of coastal
benches in northern latitudes, has been attributed to frost shattering, and
the removal of the weathered debris by wave and ice action (Andersen,
1968; Sollid et al., 1973). Rapid frost weathering has been evoked to account
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5
late glacial platform in the southwestern Scottish highlands (Gray,
1977), and it is an important erosional process on the shore platforms of
eastern Canada (Dionne, 1972; Trenhaile, 1978). The role of frost in coastal
areas, however, is poorly understood, and its significance is not universally
accepted. Guilcher (1954) suggested that saline solutions produce soft ice
which is rather ineffective for frost shattering, and Davies (1972) believed
that significant frost shattering in the intertidal zone depends upon a supply
of fresh water. Nevertheless, experimental studies, the effect of de-icing salts
on road aggregates, and of freezing sea water on coastal engineering
structures, which are well documented, show that freezing saline solutions
may have even greater competency for weathering than fresh water (Cook,
1952; Goudie, 1974). There can be little doubt therefore, that frost action
plays a very significant role in the development of platforms in high latitudes. More information, however, is necessary to determine whether these
for
a
processes produce a unique platform morphology,
prepare the coast for mechanical wave erosion.
3
or
whether
they simply
Bio-erosion
z
Bio-erosional processes have been suggested to explain the occurrence of
solutional features on tropical limestones, since the solubility of carbon
dioxide decreases in warm waters, as the degree of supersaturation with
calcium bicarbonate increases (Emery, 1946; Guilcher, 1953; Revelle and
Emery, 1957). Nocturnal lowering of temperature in rock pools, and the
release of carbon dioxide by plants and animals, lowers the Ph and causes
calcium carbonate to be converted to soluble calcium bicarbonate. This
weakens the rock cement, so that grains
be removed by waves or
may
snails.
are
ingested by
Blue-green algae
particularly effective in dissolving
limestones, and they also facilitate erosion by rock borers and browsers
(Wentworth, 1938; Yonge, 1951; Vita-Finzi and Cornelius, 1973). The
mechanical effects of bio-erosion, however, are probably far less significant
than are the effects of bio-chemical activity. Seaweed torn from platforms
by storm waves may remove slivers of the rock to which they were attached
(Everard et al., 1964). Alternatively, a mat of mussells, sea grape, and other
organisms may protect platforms from abrasion and prevent the rocks from
drying out, thereby inhibiting water layer levelling (Hills, 1949). Dense
stands of kelp may also protect platforms (Kirk, 1977), since as much as 20
per cent of a wave’s energy is required to bend artificial sea grass (Wayne,
1974).
II
The
Geology
geological
influence
on
platform development
is
expressed
in several
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6
ways:
a) Lithology and structure govern the efficiency of the processes
operating on shore platforms. If the rocks are thinly bedded and densely
jointed, wave quarrying is usually dominant, even in sheltered locations.
Rocks which are resistant to mechanical breakdown, may be more
vulnerable to chemical processes, particularly if they are capable of
absorbing large quantities of water. Geological factors may govern the
efficacy of erosional processes to such a degree that they determine the form
of platform profiles. Gill (1972b) has noted that gently sloping platforms
are associated with soft, non-soluble rocks in southeastern Australia,
whereas on highly soluble rocks, the surfaces are horizontal. The structure
and lithology of the cliff and platform are also important in determining the
type of superficial deposit at the cliff foot. The type of deposit influences the
processes operating at the cliff foot and the rate of recession. Robinson
(1977b) found that sandy beaches at the base of the cliff are most
favourable for the development of wide platforms, followed in order by a
bare cliff foot, a boulder beach, and a talus cone. This relationship has been
confirmed in south Wales, where the widest platforms are associated with a
bare cliff foot or a pebble beach, whereas the narrowest platforms are
backed by a boulder beach (Trenhaile, in preparation). This reflects the
work of abrasion associated with the presence of sand or pebbles, quarrying
where the cliff foot is bare, and the protection afforded to the cliff foot by
boulder or talus accumulations.
b) The resistance of the rocks to the processes operating on them,
determines the degree to which platforms are inherited features; this will be
discussed later.
c) Platform profiles may be strongly influenced by structural and
usually determines the surface roughness of
lithological factors. Rock
6eds
tend to provide smooth surfaces, but if dips
platforms. Gently dipping
are steep, differential erosion results in corrugated or washboard relief. The
gross form of platforms may also be influenced by geological factors. Storm
waves, splash, and spray produce supratidal ledges in gently dipping rocks
(Ferrar, 1925; Jutson, 1939; Ongley, 1940; Trenhaile, 1971). The mean
elevation of platforms increases with rock hardness (Gill, 1967; 1972b;
Trenhaile, 1969; Reffell, 1978), as does the height of the cliff-platform.
junction (Wright, 1970; Trenhaile, 1971; 1972). Theory suggests that
platforms become steeper and narrower as rock hardness increases
(Trenhaile and Layzell, 1979; and in preparation). Gradients are generally
steep on resistant rocks, but the relationship between platform width and
rock hardness may be more complex (Trenhaile, 1972; 1978). Edwards
(1941) considered that the widest platforms are cut in rocks with only
intermediate resistance to erosion, and others have noted that fairly narrow
platforms are associated with weak rocks (So, 1965; Robinson, 1977b).
Nevertheless, platforms are generally widest on unresistant rocks (Agar,
1960; Everard et al., 1964; Takahashi, 1977). The compound effects of rock
dip
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7
hardness
narrow
are
well illustrated
by
(Duckmanton, 1974),
a
the fact that high
relationship which
platforms
generally
are
usually
reflects the
presence of resistant rock.
in trying to assess the relationship between
rock hardness is to determine the strength of the
rock in relation to the processes operating on it. Gill (1967) noted that the
decrepitation of siltstones and other lithologies, renders an otherwise
resistant rock susceptible to rapid breakdown. Suzuki et al.’s (1970)
examination of washboard relief in tuff and mudstone is, unfortunately, a
unique example of the application of the techniques of rock mechanics to
the study of platform erosion. The ridges are composed of tuff, but the
abrasive hardness and the compressive and impact strengths of the
mudstones in the intervening troughs are actually greater. The relief of
the platform surface, therefore, could only be explained by the swelling
pressures and strains induced in the mudstones by water absorption.
Structural factors further complicate the assessment of rock hardness. On
washboard relief, the more resistant dipping beds stand as ridges which
protect the weaker beds in the troughs from wave attack. Resistant dipping
beds in the cliff may also form a buttress along its face, shielding the weaker
beds which lie behind. Platform width decreases as rock dip increases in
southern Japan and eastern Canada, a relationship which is consistent with
the protection afforded to the weaker beds by upstanding ridges (Trenhaile,
in preparation). The strike of dipping beds, however, should also be
considered. Everard et al. (1964) suggested that, in steeply dipping rocks,
platforms are widest when the strike is perpendicular to the cliff, whereas in
gently dipping strata they are widest when the strike is parallel to the cliff
face. Eight structural classes were used to define combinations of strike and
dip in eastern Canada and southern Japan. This confirmed theoretical
predictions that the widest platforms are associated with. horizontal rocks,
or strata which dip in a direction parallel to the cliff face, and the narrowest
platforms with vertical strata, which strike parallel or obliquely to the cliff
face (Trenhaile, in preparation).
The greatest problem
platform morphology and
.
I
A
genetic
classification
,
Shore platforms have been classified according to their stage in an
evolutionary cycle (Johnson, 1919); their morphology (Healy, 1968a; Hills,
1972; Trenhaile, 1974a), genetic considerations (Bartrum, 1935;
Wentworth, 1938); surface characteristics (Mii, 1962); and the shape, and
degree of geological control, of profiles (Robinson, 1977b). Although we
must presently depend upon inferences derived from morphological detail
to identify the processes responsible for platform development, the genetic
approach still provides the best criterion for platform classification.
Wave-cut shore platforms are the product of mechanical wave erosion,
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8
1 Platform cut in Liassic angulata limestones and shales at lVash Point
Wales. View is taken looking northwards towards Monknash Point.
Figure
Glamorgan,
but not exclusively, by storm waves. Although the genetically
’shore platform’ has replaced ’wave-cut platform’ in the
literature, the latter term is appropriate to describe a particular type
of platform. Wave-cut platforms occur in the storm wave environments
of the northern hemisphere. In Britain, they are wide ( 100-250 m),
gently sloping features ( 1-3°), which rarely terminate abruptly at
their seaward margins (So, 1965; Wright, 1967; Trenhaile, 1972;
1974a, b) (Figure 1). In Gasp6, Qu6bec, however, they are quasihorizontal, and normally terminate abruptly in a low tide cliff or ramp
(Trenhaile, 1978). Wave-cut platforms have also been reported from the
swell wave environments of Australasia (Bartrum, 1924; 1952; Johnson,
1938; Jutson, 1939; Edwards, 1941; 1951). Bartrum described these
platforms, which are found in exposed locations, as being narrow and
sub-horizontal, with abrupt seaward termini, and with mean elevations 0.3
to 2.5 m above high tide level. Wave quarrying is sensitive to geological
variations, and the platforms may have rugged surfaces. Although most
Australasian workers have acknowledged the efficacy of wave erosion in
exposed areas, many of them have maintained that, because of the
variability in wave intensity and in the levels at which waves operate,
subaerial weathering is essential for the development of horizontal surfaces
particularly,
neutral
term
(Wentworth, 1938; 1939; Hills, 1949; 1971; Hawley, 1965; Gill, 1967;
Sanders, 1968a); this most fundamental issue will be discussed later.
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9
Kaiaraara Island, Russell, Bay of Islands, New Zealand. Although originally
inspired by the shape of this island-stack, the term ’Old Hat’ has genetical connotations which
refer to platforms which are attributed to weathering of coastal cliffs, down to the level of
Figure 2
permanent saturation in the rock. The notched side of this island faces northwards, towards
the Pacific.
Dana (1849) first attributed the formation of shore platforms in warm
climates to the rapid weathering of cliffs. Dana considered that cliffs are
weathered down to the level at which the rock is permanently saturated
with sea water. He did not suggest that the platforms develop at this
saturation level, however, but that they are cut in these weathered
materials by waves, at ’the level of greatest wear’. Bell and Clarke (1909)
first proposed that the function ’of the waves is not to cut the platforms, but
rather to wash away the weathered debris, and Bartrum (1916; 1926; 1938)
considered that this process produces Old Hat platforms, as they were
termed by Hochstetter in 1864, at the level of saturation, the lowest limit to
subaerial weathering (Figure 2). Bartrum believed that Old Hat platforms
develop just below high tide level, since weak wave action is unable to
erode the unweathered rock below this saturation level, but is competent to
wash away the weathered material above. Old Hat platforms are
considered to be rather narrow, horizontal surfaces with abrupt seaward
termini. Bartrum (1916) cautioned that they are not common features,
since they require impermeable rocks which are free of joints, and sheltered
locations where wave action is feeble. Nevertheless, Old Hat platforms have
been reported from a number of areas (Edwards, 1958; Bird and Dent,
1966; Healy, 1968a; Russell, 1971). Not all workers, however, accept
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10
Bartrum’s view that mechanical wave erosion is impotent to produce Old
Hat platforms, nor that they develop near the high tide level. Mii’s (1962;
1963) hypothesis, which is reminiscent of Dana’s pioneering work, proposes
that platforms develop in the intertidal zone, where the weathered mantle is
removed by abrasion. Fairbridge (1952) claimed that the saturation level is
close to low tide level, so that Old Hat platforms which are presently near
the high tide level must be related to a former higher sea level. No work,
however, has been conducted to establish the level of saturation within
coastal cliffs. The general rejection of Fairbridge’s proposal, therefore,
appears to be a reaction to the threat posed to the only hypothesis which, it
is claimed, is capable of explaining the occurrence in fairly sheltered
environments of subhorizontal platforms near the high tide level. It would
be suprising if the saturation level is associated with any particular level,
however, since it must vary both in space and time according to climatic,
geological, and tidal factors. Bartrum carefully listed the characteristics of
Old Hat platforms, but subsequent workers have been less cautious in
ascribing this origin to shore platforms, often considering an apparently
sheltered location, or the presence of weathered cliffs to be sufficient criteria
for acceptance of the Old Hat hypothesis. The cliffs of the original Old Hat
Island (Kaiaraara or Mill Island), and adjacent areas in the Bay of Islands,
New Zealand, are much too resistant, despite their being weathered, to
envisage, as did Bartrum, the washing away of debris by weak wave action.
The essential role of wave erosion in their development is demonstrated by
the occurrence of the widest platforms on the more exposed, seaward sides
of islands. It must be repudiated that the presence of a weathered cliff
necessarily implies that a platform is of the Old Hat type. Bartrum’s
hypothesis considered that weak wave action removes fine weathered
debris, not, as many later workers have assumed, that a platform is carved
out of weathered in situ material by wave quarrying and abrasion. The
distinction, though seemingly minor, is very important. If wave erosion is
significant, the question of whether the cliff is weathered or not is of little
consequence, other than in determining rates of recession, since the
platform will develop at the level of maximum wear (Dana, 1849), rather
than at the saturation level in the rock. Evidence to be presented in this and
a forthcoming paper, suggests that the morphology of shore platforms is
generally related to tidal factors, which control the expenditure of wave
energy within the tidal range. There appears to be little evidence to support
the formation of platforms in the precise manner envisaged by Bartrum
(1916), although the weathering-abrasion hypotheses of Dana (1849) and
Mii (1962) may warrant further consideration.
Wave-cut platforms may be sufficiently modified by water layer levelling
to justify their inclusion as a separate class of platform. Bartrum and
Turner (1928) first suggested that all but the seaward portions of platforms
may be planed by subaerial weathering to produce a remarkably smooth,
horizontal surface. A residual ridge or rampart may be found at their
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11
-
Figure
3
Victoria,
Smooth
Australia.
water
layered platform
and rampart
at
Artillery Rocks,
...,._-_-
south
western
seaward margins, where it has been claimed that spray and splash keep the
rock moist, thereby preventing effective water layer levelling (Bartrum,
1935; Wentworth, 1938; Hills, 1949) (Figure 3). Some workers, however,
considered that these horizontal platforms are the product of wave erosion,
and that any ramparts which exist are simply associated with resistant rock
Johnson, 1938; Edwards, 1941; 1951; Jutson, 1949a, b; 1954; Gill, 1972c).
layered surfaces may be above or below high tide level, but Sanders
(1968a) considered that contemporary surfaces are at high tide level, and
that those below low tide or above high tide, are related to former sea
Water
levels.
The form of solutional profiles varies according to lithological, structural,
and morphogenic factors (Wentworth, 1939; Guilcher, 1953). Although
solutional features are often deeply incised into the surfaces of limestone
platforms in the storm wave environments of the northern hemisphere, it is
doubtful that solution plays a significant role in the development of these
platforms, except where the rocks are particularly resistant to wave
quarrying. Platforms which owe their very existence to solutional processes
may occur in tropical low energy environments, where wave quarrying and
abrasion are not dominant, although an alternative hypothesis, involving
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12
the work of intertidal organisms, may also be proposed to account for
’solutional nips and benches (Ginsburg, 1953; Newell and Imbrie, 1955;
Cloud, 1965).
IV
Platform
morphology and the morphogenic environment
between shore platforms in the southern and northern
hemispheres, and more particularly in Australasia and Britain, have been
the subject of much debate (Wright, 1967; Hills, 1972; Trenhaile, 1974a).
Differences
Australasian workers have noted that British and American coastal texts
generally consider only the type of shore platforms found in those countries,
thereby ignoring the occurrence of quite different forms in the southern
Pacific (Bartrum, 1926; Fairbridge, 1952; Hills, 1972). The fact that
Australasian authors have often been equally culpable in discussing
northern hemisphere platforms in a perfunctory manner is a manifestation
of the view that the two areas are morphogenetically, and therefore
morphologically distinct. It will be suggested, however, that platforms in
the two areas are related features, representing opposite ends of a tidally
regulated, wave erosional spectrum of forms.
Davies (1964) discussed the significance of differences between the
morphogenic environments of world shorelines, and Trenhaile (1974a) has
applied this concept to shore platforms. Trenhaile and Layzell (1979)
suggested that geological factors produce variations in platform geometry
about morphological means which are determined by the morphogenic
environments. Much recent work has been concerned with identifying those
factors which determine the morphology of shore platforms.
Davies (1964) emphasized differences between the wave energy
environments of world shorelines. By distinguishing between Old Hat
platforms produced by weathering in sheltered environments, and
platforms cut by waves in more exposed areas, Bartrum (1935) implicitly
related platform morphology to the wave energy of the environment.
Quantification of this relationship, however, is hindered by the lack of wave
data from rocky coastal areas. A number of workers have attempted to
relate platform width to wave energy, by qualitatively estimating the energy
environment (So, 1965; Takahashi, 1977), or by the use of wave refraction
diagrams (McLean, 1967), wave forecasting techniques (Trenhaile, in
preparation), and fetch (Flemming, 1965). Most studies have shown that
platform width is greatest on coastlines exposed to the most vigorous wave
action, an observation which is consistent with theoretical considerations
(Trenhaile and Layzell, in preparation), although several workers have
asserted that the reverse is normal (Johnson, 1933; Bartrum, 1935; Hills,
1949). This contradiction probably stems from the influence of other
factors, such as variations in cliff height, which may be negatively
correlated with platform width (Edwards, 1941; Trenhaile, 1972; 1978). It
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13
suggested that mean platform elevation increases with increasing
exposure (Duckmanton, 1974; Reffell, 1978), but cliff-platform
junctions may be higher or lower on headlands than in the adjacent
embayments, according to geological as well as wave exposure variations
along indented coastlines (Bartrum, 1935; So, 1965; Wood, 1968; Wright,
1970). Whether wave energy influences platform gradient, however,
remains to be determined. Trenhaile (1974b) did find a relationship
between platform gradient and fetch in southern Britain, but it could not be
confirmed elsewhere (Trenhaile, in preparation). Although theory suggests
that platform gradient is greatest where wave energy is low, gradient is
lowest on the less exposed parts of the Kaikoura Peninsula in southern New
Zealand (Kirk, 1977; Trenhaile and Layzell, in preparation). Despite these
contradictions, the occurrence of steeper gradients in embayments than on
the adjacent headlands (So, 1965; Hills, 1971) suggests that steep gradients
are associated with weak waves, although the effect of variable lithology
and structure along indented coastlines must always be considered. Other
aspects of platform morphology have been related to wave energy. Reffell
(1978) suggested that the height and the steepness of the low tide cliff
increases as the exposure of the coast increases, but further work is
necessary to determine whether these relationships are significant.
Davies (1964) also emphasized the significance of tidal range in
determining coastal characteristics. Cliff platform junctions or notches are
usually close to high tide level, although they are very sensitive to
geological factors (Zeuner, 1958; Wright, 1970; Trenhaile, 1972). Probably
the most fundamental relationship between platform morphology and the
morphogenic environment, however, is that between platform gradient and
tidal range (Trenhaile, 1972; 1974b; 1978). The relationship between
platform width and tidal range is more difficult to determine. It has been
suggested that width increases as the tidal range increases (Edwards, 1971;
Flemming, 1965; Wright, 1969), but attempts to quantify this relationship
have provided low or insignificant correlation coefficients in several areas
(Trenhaile, 1972; and in preparation). It is geometrically possible for
platform width to increase, decrease, or even to remain constant, as tidal
range increases, assuming that the cliff-platform junction remains at the
high tide level. Although theoretical considerations indicate that platform
width increases with the tidal range, the difficulty in confirming this
relationship in the field suggests that geology, wave energy, and other
factors exert a particularly strong influence on platform width (Trenhaile,
1978; Trenhaile and Layzell, in preparation). Other aspects of platform
morphology appear to be related to tidal factors. Tidal range may
determine the elevational range of the ramp at the cliff base, and the low
tide cliff, whereas the mean elevation of the platforms is associated with the
tidally determined level of most frequent storm wave action (So, 1965;
Trenhaile, 1972; 1978; Trenhaile and Layzell, 1979). These relationships
suggest that despite differences in climate, tectonic history, and wave
has been
wave
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14
regime, the
in response
of shore platforms around the world
variations in tidal range.
morphology
to
largely
varies
’
V
Tidal duration
Although there is a strong relationship between most aspects of platform
morphology and a number of tidal variables, in a wave dominated
environment planation must be accomplished by the waves. If the tide is
the chief architect of platform development, then it is the most vigorous or
storm waves which provide the labour.
Slight flattening of platforms about the midtide level has been attributed
to most frequent storm wave erosion at this level (So, 1965; Trenhaile,
1972). Tidal duration curves show how wave energy is distributed within
the tidal range by considering how long still water level coincides with each
intertidal elevation (Trenhaile, 1978). All tidal duration curves show that
still water level is most frequently at, or close to, midtide level, and that the
degree of concentration about this level increases with decreasing tidal
range (Figure 4).
Figure 4
Tidal duration
curves
for six
areas.
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15
The implications of tidal duration distributions were examined by using
them to simulate stages in platform development. Profiles were constructed
by calculating the recession rate at regular vertical intervals within the tidal
range. The recession rate at any level was considered to be determined by
the tidal duration value at that level, by an erodibility constant related to
deep water wave energy and rock hardness, and by the submarine gradient
(Trenhaile and Layzell, 1979). Simulated profiles attained states of
dynamic equilibrium, when the gradients at all elevations had adjusted to
their corresponding tidal duration values so as to produce uniform recession
rates across the profiles. These simulated equilibrium profiles were similar
to actual platform profiles in six study areas (Trenhaile, 1979; Trenhaile and
Layzell, in preparation). The model explained the relationship between
tidal range and platform gradient, and accounted for the prominence of
quasi-horizontal platforms with concave ramps and convex low tide cliffs in
micro and mesotidal environments. The ability of this wave erosional model
to simulate Australasian profiles suggests that wave action is able to
produce horizontal platforms. This conclusion supports the wave erosional
school Uutson, 1931; 1954; Edwards, 1941; 1951), and is contrary to those
who insist that subaerial weathering is essential for the development of
horizontal surfaces (Hills, 1949, 1971; Gill, 1967; Sanders, 1968a). A
tripartite classification, which has appeared in two Australian texts,
exemplifies the traditional neglect of the role of tidal range in determining
platform morphology (Bird, 1968; Davies, 1972). In this classification,
sloping intertidal platforms were attributed to dominant wave quarrying
and abrasion, whereas horizontal platforms, either slightly above high tide
or slightly below low tide levels, where ascribed to water layer levelling, or
to solution-biological processes, respectively. Recent work, however, has
shown that platform gradient is determined by the tidal range.
Consequently, horizontal platforms are found in Gasp6, Qu6bec, even
though water layer levelling, solution, and biological processes are
ineffective in this cool, mesotidal, storm wave environment (Trenhaile,
1978). Similarly, in the warm, macrotidal, swell wave environment of
Yampi Sound in northwestern Australia, although water layer levelling is
active, platform gradient appears to be much greater than in southern
Australia, where the tidal range is much lower (Edwards, 1958; Trenhaile,
in preparation). Inspection of platforms throughout New Zealand and
southeastern Australia, as well as published surveyed profiles, suggest that
although they are less common in the Sydney-South Coast area, horizontal
intertidal platforms close to the midtide level are more typical of
Australasia than those at the high or low tide levels. Nevertheless, variable
rock hardness and wave intensity can account for the formation of
horizontal wave-cut platforms at a variety of elevations without recourse to
subaerial weathering processes (Trenhaile and Layzell, in preparation).
The reliance on the elevation of platforms in relation to tidal levels as a
major genetic criterion in Australasia is remarkable, since few platforms
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16
have been surveyed, tidal data are imprecise in relation to terrestrial data,
and because in a micro or mesotidal environment, the margins for error are
so small. Smoothing and lowering of platforms by water layer levelling is
undoubtedly important on many Australasian platforms, but platform
morphology indicates that these processes generally only modify surfaces
whose gross form has been determined by mechanical wave action. Indeed,
it is questionable whether, in absolute terms, water layer levelling lowers
’ Australasian platforms by any more than that accomplished by corrosion
and occasionally corrasion in storm wave environments. It must be
conceded, however, that in relative terms, because of the prevalence of low
tidal ranges, the role of water layer levelling in changing the relationship
between platform elevation and specific tidal levels in Australasia is
probably greater than that of corrosion in macrotidal storm wave
environments.
VI
Inheritance, equilibrium, and
rates
of platform
development
It has been suggested that shore platforms may have been inherited from a
period when sea level was similar to today’s. Most evidence pertains to
areas composed of particularly resistant rock (Stephens, 1957; Orme, 1962;
Phillips, 1970), but inheritance has also been proposed for areas where
erosion is more rapid (Agar 1960; Dionne, 1972; Gill, 1972b). It is my
opinion, however, that shore platforms, in all but resistant rock areas, are
contemporary features in or close to a state of dynamic equilibrium. This
concept is essentially a reiteration of Edwards’s (1941), Challinor’s (1949),
and Bird’s (1968) suggestion that platform morphology may be
unchanging, as they continue to shift landwards. Evidence in support of
this hypothesis includes the close relationships which have been found
between aspects of platform morphology, wave energy, and tidal range. In
theory, a negative feedback loop, involving the relationship between the
wave energy reaching the coast and platform gradient and width, makes it
inevitable that platforms will eventually attain states of dynamic
equilibrium (Trenhaile, 1974b). This has been confirmed by two theoretical
models. The first model, which has already been discussed, considers the
tidally controlled expenditure of wave energy within the tidal range,
whereas the second is concerned with the time required to undercut the cliff
to the point of collapse and to remove the debris produced (Trenhaile, in
preparation). Both models produce equilibrium profiles which are similar
to actual platform profiles in several areas. Although the amount of time
required to attain equilibrium in the models is sensitive to several
assumptions, it is worth noting that within 5000 years most simulated
profiles had achieved, or were very close to achieving, their equilibrium
forms and in many cases little change occurred after about 2500 years.
Although these figures suggest that shore platforms have developed since
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17
sea reached its present level, whether this is actually the case depends
upon the rates of cliff and platform erosion. Reported rates of platform
downwasting range between 1 X 10-4 and 3.5 X 10-’ m yr-’, being greatest
in exposed wave-dominated environments. Backwasting rates, which are
generally much greater, range from the negligible to as much as 50-75 m
yr-’ (Sunamura, 1973; Kirk, 1977). Many estimates of erosion rates,
however, are based upon sequential terrestrial and aerial photography, old
maps, and other techniques which are of doubtful accuracy. Furthermore,
many downcutting estimates refer to the contributions of single agencies,
the
such
as
bio-erosion (Healy, 1968b; Evans, 1968), or chemical processes
(Revelle and Emery, 1957), rather than to the gross rates of lowering. The
micro-erosion meter has recently provided some valuable information on
rates of platform lowering (Kirk, 1977; Robinson, 1977a; b; c). There is,
however, a danger that the limitations of this technique to measure gross
rates of platform downwasting may be overlooked. Design changes (Gill,
personal communication) will improve and provide checks on the
measurement accuracy of the instrument, but they will not affect the
inherent inability of the technique to consider the quarrying of large rock
fragments, a process which is clearly dominant in many if not most areas.
Variations in postglacial climate, together with the very rapid rates of
platform erosion during the initial development stages (Trenhaile and
Layzell, 1979; and in preparation), makes it difficult to extrapolate
short-term erosion rates to longer periods. Nevertheless, since
contemporary erosion rates are probably lower than they were in the past,
relatively rapid erosion at present indicates that platforms could have
developed within a few thousand years (Kirk, 1977). This view is consistent
with Takahashi’s (1977) estimate that most Japanese platforms have taken
about 3000 years to develop. Therefore, although there is ample evidence of
inheritance in areas with particularly resistant rocks, the suggestion that
shore platforms in less resistant lithologies are inherited is less tenable.
VII
Interpretation of raised platforms
b
Despite the obvious relationship between shore platforms developing at
present sea level, and raised platforms related to a higher relative level of
the sea, discussion of these latter features rarely encompasses consideration
of their contemporary counterparts. It is beyond the scope of this review to
consider the interpretation of raised surfaces, but it is pertinent to emphasize
that relationships between contemporary platform morphology, geology,
and aspects of their morphogenic environments may be relevant to
determining the sea level, its stability, and the morphogenic environment
during the formation of raised platforms.
!
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18
VIII
Future work
Platforms in most of the American, African, and Asian continents have yet
be discussed, and even in Britain and Australasia little work has been
conducted in sparsely populated areas. Of particular value would be studies
in cool, microtidal, storm wave and warm, macrotidal, swell wave
environments (Edwards, 1958; Trenhaile, 1978), where the relative
significance of wave erosion and weathering could be further evaluated.
Few studies have been concerned with measuring platform erosion rates.
The microerosion meter may now be used where wave quarrying of large
rock fragments is insignificant, but the processes responsible for platform
downwearing must still be inferred from the erosional data. We certainly
need to know more about the processes operating on shore platforms, but it
must be appreciated that measurement in the field pertains only to the
processes presently acting upon platforms, rather than to the processes
originally responsible for their planation. Wave quarrying in particular
would probably have been even more effective during the early stages of
platform development, when the profiles were steep and irregular, than it is
on
many of the smooth, level surfaces of today. Theoretical and
experimental work may provide some insight into these early stages.
Finally, it should be noted that little work has been conducted on the
submarine extensions of shore platforms. Information on submarine
processes and their rates of operation are required to determine the
long-term development of shore platforms, and to assess the relative
contributions of the intertidal and offshore zones to the formation of very
wide raised platforms.
to
University of Windsor,
IX
Ontario
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