Geology Reference
In-Depth Information
5
The extent to which Pliocene marine transgressions
invaded the lower and flatter portions of the Late
Neogene land surface remains controversial. There
exists evidence for a Red Crag (Pre-Ludhamian)
incursion that affected the London Basin and east-
ern parts of the Weald, but growing uncertainty as
to the validity of an earlier Lenham Beds incursion.
The Red Crag incursion appears the consequence
of relatively localized downwarping, and is consid-
ered to have caused minimal erosion, but sufficient
to disrupt the surface duricrusts.
(Preece
et al.
1990). This uplift, in conjunction
with oscillating sea levels and climatic fluctuations,
resulted in episodic erosion that increased in scale
as relative relief developed through the Pleistocene.
Jones (1999) owns that this evolutionary model
needs substantiating in a number of important regards.
Uncertainty surrounds the true nature of the struc-
tural foundations of the area and its tectonic evolution,
which fuels the continuing controversy over the tem-
poral and spatial dimensions of uplift and the relative
importance of Mid-Tertiary (Miocene) tectonic activ-
ity. Likewise, the recent suggestions of Pleistocene
uplift and warping need confirming, elaborating, and
accurately dating. Moreover, the nature and palaeo-
environmental significance of residual soils, including
the varied types of silcrete, and the number, age, and
geographical extent of Neogene marine incursions,
most especially the baffling Lenham Beds incursion,
still demand much investigation. Only after they com-
plete this further work will geomorphologists be able to
establish a fully detailed evolutionary geomorphology
for this well-known region.
6
The subsequent marine regression revealed a gen-
tly inclined marine plain that suffered lowering
through etchplanation that fashioned the promi-
nent 'platforms' exposed on the flanks of the
London Basin.
7
At some time after 2 million years ago, further
uplift and warping occurred, possibly as discon-
tinuous pulses through much of the Pleistocene
epoch. While eastern East Anglia suffered subsi-
dence, the remainder of southern England experi-
enced differential uplift to a maximum of at least
250 m (Jones 1999b) and possibly up to 400 m
ANCIENT LANDFORMS
all the landforms in a landscape. Far more commonly,
remnants of past landforms are preserved. Consequently,
most landscapes are a complex collection of landforms
inherited from several generations of landscape develop-
ment, a palimpsest of past forms. In some landscapes,
the inherited forms were fashioned by processes similar
to those now operating there, but it is common to find
polygenetic soil landscapes in which the processes respon-
sible for a particular landform no longer operate. The
clearest and least equivocal example of this is the glacial
and periglacial landforms left as a vestige of the Ice Age
in mid-latitudes. Other polygenetic landscapes are com-
mon. In deserts, ancient river systems, old archaeological
sites, fossil karst phenomena, high lake strandlines, and
deep weathering profiles are relict elements that attest
to past humid phases; while stabilized fossil dune fields
Relict landforms
'Little of the earth's topography is older than the Tertiary
and most of it no older than Pleistocene' (Thornbury
1954, 26). For many decades, this view was widely held
by geomorphologists. Research over the last twenty years
has revealed that a significant part of the land surface is
surprisingly ancient, surviving in either relict or buried
form (see Twidale 1999).
Relict landscapes
and
land-
forms
endure for millions, tens of millions, or hundreds
of millions of years.
It is common for a cliff, a floodplain, a cirque, and
many other landscape features to survive longer than
the climatic regime that created them. Seldom does the
erosion promoted by a new climatic regime renew
