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surface in north Wales, one of the highest and on Davis's
terms therefore one of the oldest in Britain. Modern
evidence suggests that it is a lowland surface, uplifted in
mid to late Tertiary times by sustained ridge-push forces
on the European continental margin as the Atlantic
continued to widen. It was subsequently breached by
Quaternary glaciation and the region is still seismically
active.
Denudation cycles and chronologies are now largely
discredited but they were formulated long before sea-
floor spreading, volcano-tectonic activity and the
dramatic events of the Quaternary were understood.
These clarify the complex nature and rates of uplift and
denudation, the speed and frequency of disturbances
to denudational 'rhythms', and means of dating events.
Older models projected whole landsystem effects without
comprehending the total landsystem and were rather
parochial. Davis's 'normal cycle' assumed fluvial con-
ditions in temperate environments where glaciation was
seen as a climatic 'accident' and King worked largely in
semi-arid southern Africa. However, Davis recognized the
progressive transformation of potential to kinetic energy
as his cycle proceeded and his graded river profile reflects
exponential energy decay and the progressive reduction
of elevation, relief and slope angles.
Force and resistance
Debate shifted away from denudation cycles to explore the
validity of a morphogenetic basis for modern geomor-
phology. Its pervasive presumption of climatic control
on landform assemblages led to the creation of morpho-
genetic maps ( Figure 13.3 ). At first sight they are plaus-
ible, especially since glacial and hot desert environments
equate with modern climatic zones. Climate inconstancy
and tectonic displacement of crust across climatic
zones cannot be ignored, however. Many landforms are
polygenetic , formed by more than one process. Close
affinities with climate also recede when we consider that
many Pleistocene glacier landsystems now lie outside the
glacial zone and are reminders of fluctuating climate
patterns. Many landforms are not specific to any one
climatic zone. Slopes and fluvial channels evolve in all
climates, and coastlines and mountains are distinctly
LONGITUDE
60°
120°
180°
120°
60°
80°
80°
60°
60°
40°
40°
20°
20°
20°
20°
40°
40°
60°
60°
60°
120°
180°
120°
60°
Glaciers and ice-sheets
Permafrost
Other periglacial
Humid mid-latitude
Mediterranean
Tropical semi-arid
Tropical seasonally dry
Hot desert
Humid tropical
Mountains
Arid cold winters
Figure 13.3 Global morphogenetic regions, based on morphoclimatic zones.
Source: After Tricart and Cailleux (1972)
 
 
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