Geology Reference
In-Depth Information
on desert margins are relicts of more arid phases. In the
humid tropics, a surprising number of landscape features
are relict. For example, researchers working in the central
Amazonian Basin (Tricart 1985) and in Sierra Leone
(Thomas and Thorp 1985) have unearthed vestiges of
fluvial dissection that occurred under dry conditions
between about 20,000 and 12,500 years ago.
of the Earth extended much further polewards than it
does today. Indeed, evidence from deposits in the soil
landscape, like evidence in the palaeobotanical record,
indicates that warm and moist conditions extended to
high latitudes in the North Atlantic during the late
Cretaceous and Palaeogene periods. Julius Büdel (1982)
was convinced that Europe suffered extensive etchplana-
tion during Tertiary times. Traces of a tropical weathering
regime have been unearthed (e.g. Battiau-Queney 1996).
In the British Isles, several Tertiary weathering products
and associated landforms and soils have been discov-
ered (e.g. Battiau-Queney 1984, 1987). On Anglesey,
which has been a tectonically stable area since at least
the Triassic period, inselbergs, such as Mynydd Bodafon,
have survived several large changes of climatic regime
(Battiau-Queney 1987). In Europe, Asia, and North
America many karst landscapes are now interpreted as
fossil landforms originally produced under a tropical
weathering regime during Tertiary times (Büdel 1982;
Bosák
et al
. 1989).
Relict land surfaces
In tectonically stable regions, land surfaces, especially
those capped by duricrusts, may persist a hundred
million years or more, witness the Gondwanan and
post-Gondwanan erosion surfaces in the Southern
Hemisphere (King 1983). Some weathering profiles in
Australia are 100 million years old or even older (Ollier
1991, 53). Remnants of a ferricrete-mantled land surface
surviving from the early Mesozoic era are widespread
in the Mount Lofty Ranges, Kangaroo Island, and the
south Eyre Peninsula of South Australia (Twidale
et al
.
1974). Indeed, much of south-eastern Australia con-
tains many very old topographical features (Young 1983;
Bishop
et al
. 1985; Twidale and Campbell 1995). Some
upland surfaces originated in the Mesozoic era and
others in the early Palaeogene period; and in some
areas the last major uplift and onset of canyon cut-
ting occurred before the Oligocene epoch. In southern
Nevada, early to middle Pleistocene colluvial deposits,
mainly darkly varnished boulders, are common features
of hillslopes formed in volcanic tuff. Their long-term sur-
vival indicates that denudation rates on resistant volcanic
hillslopes in the southern Great Basin have been exceed-
ingly low throughout Quaternary times (Whitney and
Harrington 1993).
In Europe, signs of ancient saprolites and duricrusts,
bauxite and laterite, and the formation and preserva-
tion of erosional landforms, including tors, inselbergs,
and pediments, have been detected (Summerfield and
Thomas 1987). A kaolin deposit in Sweden has Creta-
ceous oysters in their growth positions on corestones.
These corestones must have been weathered, eroded,
and exposed on a beach by around 80 million years
ago (Lidmar-Bergström 1988). The palaeoclimatic sig-
nificance of these finds has not passed unnoticed: for
much of the Cenozoic era, the tropical climatic zone
Relict glacial and periglacial landforms
Many of the glacial landforms discussed in Chapter 10
are relicts from the Pleistocene glaciations. In the English
Lake District, U-shaped valleys, roches moutonnées, stri-
ations, and so on attest to an icy past. However, not all
signs of glaciation are incontrovertible. Many landforms
and sediments found in glaciated regions, even those
regions buried beneath deep and fairly fast-flowing ice,
have no modern analogues. Landforms with no modern
analogues include drumlins, large-scale flutings, rogens,
and hummocky topography. This means that drumlins
are not forming at present, and the processes that fashion
them cannot be studied directly but can only be inferred
from the size, shape, composition, and location of relict
forms.
Glacial landforms created by Pleistocene ice may be
used as analogues for older glaciations. For instance,
roches moutonnées have been found in the geological
record. Abraded bedrock surfaces in the Neoprotero-
zoic sequence of Mauritania contain several well-
developed ones, and others have been found in the Late
Palaeozoic Dwykas Tillite of South Africa (Hambrey
1994, 104).
