Geoscience Reference
In-Depth Information
20
North American deserts
Climb the cliff at the end of Labyrinth Canon, and look over the plain
below, and you see vast numbers of buttes scattered about over scores
of miles, and every butte so regular and beautiful that you can hardly
cast aside the belief that they are works of Titanic art.
John Wesley Powell (1834-1902)
Explorations of the Colorado River of the West
(1875)
20.1 Introduction
Although the individual deserts in North America and northern Mexico are quite
small when compared to the great deserts of the Sahara, Arabia, Australia and Asia,
no other continent has had such an enormous impact on global climatic changes
throughout the Quaternary as North America. Great ice caps waxed and waned in the
north of the continent and had a profound influence on global sea level and global
oceanic and atmospheric circulation (see
Chapter 3
). The most recent of these was the
Laurentide ice cap - so-named because it was located over the Laurentian shield. This
ice cap was up to 3 km thick and reached its maximum extent as recently as 20,000
years ago. As a result of the vast amounts of water locked up in the ice caps of North
America, Greenland and north-west Europe during the Last Glacial Maximum, global
sea levels fell by 120 m, exposing hitherto submerged continental shelves (Chappell,
1974
; Chappell et al.,
1996
; Lambeck and Chappell,
2001
). Air temperatures to the
south and west of the Laurentide ice cap were so low throughout the year that the
ground was permanently frozen except along the southern margins of the permafrost
region, which thawed out briefly at the surface during the short summers. It was
not until the ice caps began to melt and a corridor opened up between the shrinking
Cordilleran ice cap to the west and the Laurentide ice cap to the east that prehistoric
hunters were able to enter North America from eastern Siberia across the Bering
land bridge and down the Mackenzie Valley some 14,000 years ago, although some
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