ABSTRACT
The findings of historical geology are very clear that during the last 1,000 million years, for which good stratigraphical evidence is available, world climates have undergone several periods of great change, some of long duration but others much shorter. At the two extremes we have on the one hand the ice ages, when areas of continental dimensions were covered with ice sheets and mountain glaciers were greatly enlarged, and on the other, periods when tropical climates extended as far as 50-60° north and south of the equator and the polar regions were not ice-covered as they now are. In this changing scene the deserts of today were at times well-watered lands, and again, regions at present well watered were once deserts. No account of the existing arid areas can omit reference to the relatively recent climatic changes of the Quaternary era, which include several ice ages and interglacial periods, the effects of which on life and on geomorphology and soils in arid lands, as elsewhere, are still with us; but even the older events of the Tertiary era are also of direct significance, since fossil soils of Tertiary age are still preserved in the present land surface, especially in arid regions, and many major geological and geomorphological features were initiated during the Tertiary. In interpreting the record of the rocks as to climates during geological time, the conditions of present deserts afford an analogue for aridity. Reference will be made to-this in later paragraphs, but the complexity of the topic is best seen by reference to the inferred changes that have affected existing arid areas, and evidence for the extension of contraction of the arid areas during recent times.
