North American deserts - Climatic Change in Deserts - page 387

Figure 20.3. Desert landscape, Arizona.
to produce peneplains of the type espoused by William Morris Davis ( 1909 ; 1912 )
and later popularised by the great New Zealand geomorphologist Sir Charles Cotton
( 1947 ), who considered desert landscapes to be 'climatic accidents' from the so-
called 'normal cycle of erosion'. Such debates ultimately proved sterile, given that
the opposing conclusions attained by the different advocates are in fact scientifically
untestable and arise simply from the initial unproven assumptions implicit in each
proposed model of landscape evolution. Be that as it may, the public and scientific
focus on desert landscapes prompted careful attention to the processes of weathering
and erosion in arid regions and the recognition that present-day desert landscapes
are polygenic, having been fashioned over many millions of years under a variety
of different climates (Mabbutt, 1977 ; Frostick and Reid, 1987a ;Cookeetal., 1993 ;
Thomas, 1997 ; Parsons and Abrahams, 2009 ). With these general ideas in mind, let
us now consider where the North American deserts fit into the major landscapes of
that continent.
Put very simply, North America consists of three main geomorphic elements: the
western mountains, the central lowlands and the eastern hills ( Figure 20.4 ). Each may
be subdivided into distinct smaller units (Thornbury, 1965 ; Graf, 1987b ). From west
to east, the western mountains include the Coastal Ranges and Sierra Nevada, the
High Plateaux, and the Northern and Southern Rockies. The High Plateaux include
the Lava Plains in the north, the Great Basin in the centre, and the Colorado Plateau