Deserts of the World - Design with the Desert: Conservation and Sustainable Development - page 25

caused by crustal extension, either because high heat flow above the subducted plate may
have caused doming and extension of the crust or because the subducting plate may have
broken, causing very high heatflow and extension of the crust for a limited period.
These tectonic events provided the setting for some of the most important geomor-
phological features of the arid west playas, although deflation and other processes have
contributed to their development and form, they are essentially products of a particular
tectonic setting. That these mainly dry playa basins formerly contained large lakes was
documented around a century ago. 114 It is now recognized that more than a 100 closed
basins in the western United States contained lakes during the late Wisconsin, but only
about 10% of them are perennial and of substantial size today. The largest of the basins
was Bonneville, which had a length of around 313 miles, a maximum area of 20,300 miles 2 ,
and a depth of about 1,100 ft. Lake Lahontan, the second largest lake, had an area of 9000
miles 2 and a 918 ft maximum depth.
The dating of hydrological changes in these basins over the past 40,000 years is relatively
precise, 115 and the lake-level curves from a number of basins show a considerable degree
of similarity. It is strikingly evident that from c. 24,000 to 14,000 year BP (i.e., more or less
contemporaneous with the maximum of the last Wisconsin, glaciation) that lake levels
were high. There was then a phase when lake levels showed marked fluctuations, before
a period of drought between 10,000 and 5,000 year BP, which culminated between 6,000
and 5,000 year BP. While the temporal patterns of the fluctuations may be relatively well
known, the explanation for the greatly expanded pluvial lakes is more controversial, and
there has been a longstanding debate as to whether the crucial control was the diminished
evapotranspirational losses brought about by late Pleistocene temperature reductions or
some increase in precipitation levels. 116
Although the pluvial legacy is widely evident in the presence of old lake beds, high lake
shorelines, and formerly integrated fluvial systems, elsewhere there is abundant evidence
for the formerly greater power and distribution of aeolian processes, as revealed by the
presence of aligned drainage systems, yardangs, shaped depressions, and associated
lunettes and, most importantly of all, of palaeodune systems. These were first recognized
by Price 117 and have recently been mapped in a great belt of country in the lee of the
western cordillera between the Canadian Line and the Gulf of Mexico. 118 The dating of
the largely relict dune fields, of which the Nebraskan Sandhills are the largest example, 119
is controversial. Debate surrounds the relative importance of late Pleistocene and mid-
Holocene (altithermal) arid phases. 120 The stratigraphy, sedimentology, and arrangement
of lunettes in the lee of large deflation basins cut into palaeodrainage systems on the High
Plains of Texas indicate that there were multiple phases of aeolian activity. Indeed, aeolian
activity in the High Plains, as represented by the Blackwater Draw Formation, dates back
to beyond 1.5 million year BP. 121
1.10 Conclusion
The world's deserts show the imprint of climatic changes at many different time scales.
These range from runs of a few dry or wet years, through to phases of the duration of a
decade or decades (e.g., the Dust Bowl years of North America in the 1930s or the persistent
drought of the Sudano-Sahel belt since the late 1960s), through to extended periods of
some centuries or millennia (e.g., the dry hypsithermal phase of the American High Plains