Page 23 - Fundamentals of Geomorphology

6 INTRODUCING LANDFORMS AND LANDSCAPES

studies but are making a strong comeback. Although terms, borrowed from biology, are misleading and much
process and historical studies dominate much modern censured (e.g.Ollier 1967; Ollier andPain1996,204–5).
geomorphological enquiry, particularly in English- The ‘geographical cycle’ was designed to account for the
speaking nations, other types of study exist. For exam- development of humid temperate landforms produced
ple, structural geomorphologists, who were once a by prolonged wearing down of uplifted rocks offering
very influential group, argued that underlying geological uniform resistance to erosion. It was extended to other
structures are the key to understanding many landforms. landforms, including arid landscapes, glacial landscapes,
Climatic geomorphologists, who are found mainly periglacial landscapes, to landforms produced by shore
in France and Germany, believe that climate exerts a processes, and to karst landscapes.
profound influence on landforms, each climatic region William Morris Davis’s ‘geographical cycle’ – in which
creating a distinguishing suite of landforms (p. 13). 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work, even if it has been superseded (Figure 1.2).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
Historical geomorphology
appeal seems to have lain in its theoretical tenor and
Traditionally, historical geomorphologists strove to work in its simplicity (Chorley 1965). It had an all-pervasive
out landscape history by mapping morphological and influence on geomorphological thought and spawned the
sedimentary features. Their golden rule was the dictum once highly influential field of denudation chronology.
that ‘the present is the key to the past’. This was a The work of denudation chronologists, who dealt mainly
warrant to assume that the effects of geomorphic pro- withmorphologicalevidence,wassubsequentlycriticized
cesses seen in action today may be legitimately used to for seeing flat surfaces everywhere.
infer the causes of assumed landscape changes in the
past. Before reliable dating techniques were available, Walther Penck
such studies were difficult and largely educated guess-
work. However, the brilliant successes of early historical A variation on Davis’s scheme was offered by Walther
geomorphologists should not be overlooked. Penck. According to the Davisian model, uplift and pla-
nation take place alternately. But, in many landscapes,
William Morris Davis uplift and denudation occur at the same time. The con-
tinuous and gradual interaction of tectonic processes and
The ‘geographical cycle’, expounded by William denudation leads to a different model of landscape evo-
Morris Davis, was the first modern theory of land- lution, in which the evolution of individual slopes is
scape evolution (e.g. Davis 1889, 1899, 1909). It thought to determine the evolution of the entire land-
assumed that uplift takes place quickly. Geomorphic scape (Penck 1924, 1953).Three main slope forms evolve
processes, without further complications from tectonic with different combinations of uplift and denudation
movements, then gradually wear down the raw topog- rates. First, convex slope profiles, resulting from wax-
raphy. Furthermore, slopes within landscapes decline ing development (aufsteigende Entwicklung), form when
through time – maximum slope angles slowly lessen the uplift rate exceeds the denudation rate. Second,
(though few field studies have substantiated this claim). straight slopes, resulting from stationary (or steady-state)
So topography is reduced, little by little, to an exten- development (gleichförmige Entwicklung), form when
sive flat region close to baselevel – a peneplain – uplift and denudation rates match one another. And,
with occasional hills, called monadnocks after Mount third, concave slopes, resulting from waning develop-
Monadnock in New Hampshire, USA, which are local ment (absteigende Entwicklung), form when the uplift
erosional remnants, standing conspicuously above the rate is less than the denudation rate. Later work has
general level. The reduction process creates a time shown that valley-side shape depends not on the simple
sequence of landforms that progresses through the interplay of erosion rates and uplift rates, but on slope
stages of youth, maturity, and old age. However, these materials and the nature of slope-eroding processes.