Geoscience Reference
In-Depth Information
Figure 20.1. Horizontal bedding of rocks exposed by erosion, Grand Canyon,
Arizona.
earlier arrivals by other coastal routes cannot be ruled out. Whether the arrival of these
hunters was the primary cause of the late Pleistocene and early Holocene extinctions
of the North American megafauna is still a matter of vigorous debate (see
Chapter 17
).
The aim of this chapter is to describe the environmental fluctuations experienced
by the drier parts of North America, bearing in mind that many areas that are now
relatively humid were previously arid, just as many of the arid areas were once far
wetter than they are today.
20.2 Desert landscapes of North America
The desert landscapes of North America are among the best-known landscapes on
earth (Thornbury,
1965
; Graf,
1987b
)(
Figures 20.1
to
20.3
). The detailed three-
dimensional sketches of John Wesley Powell, Grove Karl Gilbert and many other
intrepid geological explorers of the mid-nineteenth century brought to public atten-
tion the monumental grandeur and mystery of such features as the Grand Canyon
(see
Chapter 5
). Attempts to explain the origin of these unique landforms resulted
in the formulation of some of the most fundamental concepts and principles of geo-
morphology, including notions of river base level, stream antecedence, superposition,
competence and capacity (see
Chapter 10
; Baulig,
1950
; Chorley et al.,
1964
;Sparks,