Geology Reference
In-Depth Information
marine activity, Wooldridge and Linton interpreted it
as a Pliocene marine plain. Wooldridge and Linton
believed that the two higher surfaces - the Summit
Surface and the marine platform - were not warped.
They argued, therefore, that these surfaces must have
formed after the tectonic episode that deformed the
Sub-Eocene Surface, and that the summit plain had
to be a peneplain fashioned during the Miocene and
Pliocene epochs.
The Wooldridge and Linton model of Tertiary
landscape evolution was the ruling theory until at
least the early 1960s and perhaps as late as the early
1970s. Following Wooldridge's death in 1963, inter-
est in the long-term landform evolution of Britain -
or denudation chronology, as many geomorphologists
facetiously dubbed it - waned. Critics accused denuda-
tion chronologists of letting their eyes deceive them:
most purely morphological evidence is 'so ambiguous
that theory feeds readily on preconception' (Chorley
1965, 151). However, alongside the denigration of
and declining interest in denudation chronology, some
geomorphologists reappraised the evidence for long-
term landscape changes. This fresh work led in the
early 1980s to the destruction of Wooldridge and
Linton's 'grand design' and to the creation of a new
framework that discarded the obsession with morpho-
logical evidence in favour of the careful examination
of Quaternary deposits (Jones 1999, 5-6). The reap-
praisal was in part inspired by Philippe Pinchemel's
(1954) alternative idea that the gross form of the Chalk
backslopes in southern England and northern France
results from intersecting Palaeogene erosion surfaces
that suffered exhumation and modification during the
Neogene and the Quaternary times. Foremost among
the architects of the new model of Tertiary landscape
evolution in southern England were Jones (1981) and
Green (1985). Jones (1999) confirmed this model in
a region-wide synthesis, which, paraphrasing Jones's
explanation, runs thus (Figures 15.5 and 15.6):
Cretaceous Chalk ceased in the Maastrichtian, and
dry land had probably emerged by 65 million
years ago.
2
Palaeocene denudation rapidly stripped up to
350 m of Chalk, with the severest erosion on
such uplift axes as the Weald and the Channel
High, and in the west, where subaerial denudation
under tropical climatic conditions quickly removed
most of a sizeable Chalk layer. A combination
of eustatic fluctuations and tectonic movements
led to a progressive encroachment of marine con-
ditions from the east, starting with the Thanet
Sands during the Palaeocene epoch around 57 mil-
lion years ago, and ending in a nearly complete
inundation by the London Clay sea in the Early
Eocene epoch, some 53 million year ago (Pomerol
1989). The Palaeocene and Early Eocene sedi-
ments accumulated on a multifaceted or polycyclic,
marine-trimmed Sub-Palaeogene Surface cut in
Chalk. The only exception is the extreme west,
where the Upper Greensand had been exposed
by the close of the Palaeocene epoch beneath a
widespread etchplain, the lower parts of which
were easily submerged by the transgressing London
Clay sea.
3
Continuing pulses of tectonic deformation
throughout the remainder of the Palaeogene
period saw the further definition of the struc-
tural basins (London and Hampshire-Dieppe
Basins) due to the progressive growth of the Weald-
Artois Anticline and the Isle of Wight Monocline.
Sub-aerial erosion on the axes of these upwarps
led to the development of further facets of the
Sub-Palaeogene Surface, while sedimentation was
progressively limited to the basin areas and ulti-
mately restricted to the Hampshire Basin in the
Oligocene. Elsewhere, sub-aerial erosion under
the hot climatic conditions of the Eocene epoch
created an extensive etchplain with duricrusts over
most of the present Chalklands that sat at an
elevation a few tens of metres above the highest
present summits. In the west, this surface had orig-
inated in the Palaeocene epoch and slowly evolved
1
As a result of a combination of a eustatic fall in sea-
level and tectonic deformation, the deposition of a
continuous and thick (up to 550 m) sheet of Upper
