Geology Reference
In-Depth Information
Erosion surfaces became unfashionable, particularly in
British geomorphological circles, during the second half
of the twentieth century, with many geomorphologists
questioning their existence. However, the current con-
sensus is that they do exist, and a revival of interest in
them is apparent. As Ollier (1981, 152) not so tactfully
put it, 'Most people who are not blind or stupid can tell
when they are in an area of relatively flat country: they
can recognize a plain when they see one'. Of course, a
plain may be depositional, constructed from successive
layers of alluvial, lacustrine, marine, or other sediments.
Erosional plains cut across diverse bedrock types, and
geological structures are all planation surfaces of some
kind. They occur in low-lying areas and at elevation.
Elevated plains that sometimes bear signs of an erosional
origin followed by subsequent dissection. A good exam-
ple is a
bevelled cuesta
. Here, the flat top or bevel on a
cuesta is credible evidence that an upper erosion surface,
sitting at about the level of the bevel, existed before dif-
ferential erosion moulded the cuesta. A word of warning
is in order here: one bevelled cuesta does not a plana-
tion surface make. An isolated bevel might have been
a river terrace or some other small flat feature. Only
when many bevelled cuestas occur, with the bevels all
at about the same elevation, does the former existence
of a planation surface seem likely. A
shelf
is produced
if planation fails to remove the entire top of a cuesta
and instead erodes a bench. A much discussed exam-
ple is the early Pleistocene bench on the North Downs
and Chiltern Hills of England. Plateaux are also elevated
plains.
Peneplains
The Davisian system of landscape evolution (p. 6) con-
sists of two separate and distinct cyclical models, one for
the progressive development of erosional stream valleys
and another for the development of whole landscapes
(Higgins 1975). Valleys are thought to be V-shaped in
youth, flat-bottomed in maturity, after lateral erosion
has become dominant, and to possess very shallow fea-
tures of extensive plains in old age, after lateral erosion
has removed all hills (Figure 15.1). Young landscapes
are characterized by much flat topography of the orig-
inal uplifted peneplain. Mature landscapes have deeper
and wider V-shaped valleys that have consumed much of
the interfluves bearing remnants of the original land sur-
face. Old landscapes are characterized by a
peneplain
,in
which the interfluves are reduced to minor undulations
(Figure 15.1).
Figure 15.1
Traditional Davisian stage names for valley profiles and for landscape profiles.
Source:
Adapted from Ollier and Pain (1996, 204, 205)
