Geology Reference
In-Depth Information
studies but are making a strong comeback. Although
process and historical studies dominate much modern
geomorphological enquiry, particularly in English-
speaking nations, other types of study exist. For exam-
ple,
structural geomorphologists
, who were once a
very influential group, argued that underlying geological
structures are the key to understanding many landforms.
Climatic geomorphologists
, who are found mainly
in France and Germany, believe that climate exerts a
profound influence on landforms, each climatic region
creating a distinguishing suite of landforms (p. 13).
terms, borrowed from biology, are misleading and much
censured (e.g. Ollier 1967; Ollier and Pain 1996, 204-5).
The 'geographical cycle' was designed to account for the
development of humid temperate landforms produced
by prolonged wearing down of uplifted rocks offering
uniform resistance to erosion. It was extended to other
landforms, including arid landscapes, glacial landscapes,
periglacial landscapes, to landforms produced by shore
processes, and to karst landscapes.
William Morris Davis's 'geographical cycle' - in which
landscapes are seen to evolve through stages of youth,
maturity, and old age - must be regarded as a classic
work, even if it has been superseded (Figure 1.2). Its
appeal seems to have lain in its theoretical tenor and
in its simplicity (Chorley 1965). It had an all-pervasive
influence on geomorphological thought and spawned the
once highly influential field of denudation chronology.
The work of denudation chronologists, who dealt mainly
with morphological evidence, was subsequently criticized
for seeing flat surfaces everywhere.
Historical geomorphology
Traditionally, historical geomorphologists strove to work
out landscape history by mapping morphological and
sedimentary features. Their golden rule was the dictum
that '
the present is the key to the past
'. This was a
warrant to assume that the effects of geomorphic pro-
cesses seen in action today may be legitimately used to
infer the causes of assumed landscape changes in the
past. Before reliable dating techniques were available,
such studies were difficult and largely educated guess-
work. However, the brilliant successes of early historical
geomorphologists should not be overlooked.
Walther Penck
A variation on Davis's scheme was offered by
Walther
Penck
. According to the Davisian model, uplift and pla-
nation take place alternately. But, in many landscapes,
uplift and denudation occur at the same time. The con-
tinuous and gradual interaction of tectonic processes and
denudation leads to a different model of landscape evo-
lution, in which the evolution of individual slopes is
thought to determine the evolution of the entire land-
scape (Penck 1924, 1953). Three main slope forms evolve
with different combinations of uplift and denudation
rates. First, convex slope profiles, resulting from wax-
ing development (
aufsteigende Entwicklung
), form when
the uplift rate exceeds the denudation rate. Second,
straight slopes, resulting from stationary (or steady-state)
development (
gleichförmige Entwicklung
), form when
uplift and denudation rates match one another. And,
third, concave slopes, resulting from waning develop-
ment (
absteigende Entwicklung
), form when the uplift
rate is less than the denudation rate. Later work has
shown that valley-side shape depends not on the simple
interplay of erosion rates and uplift rates, but on slope
materials and the nature of slope-eroding processes.
William Morris Davis
The '
geographical cycle
', expounded by
William
Morris Davis
, was the first modern theory of land-
scape evolution (e.g. Davis 1889, 1899, 1909). It
assumed that uplift takes place quickly. Geomorphic
processes, without further complications from tectonic
movements, then gradually wear down the raw topog-
raphy. Furthermore, slopes within landscapes decline
through time - maximum slope angles slowly lessen
(though few field studies have substantiated this claim).
So topography is reduced, little by little, to an exten-
sive flat region close to baselevel -
a peneplain
-
with occasional hills, called
monadnocks
after Mount
Monadnock in New Hampshire, USA, which are local
erosional remnants, standing conspicuously above the
general level. The reduction process creates a time
sequence of landforms that progresses through the
stages of
youth
,
maturity
, and
old age
. However, these
