PHYSICAL DEVELOPMENT OF CENTRAL ASIA 645 



ing regional precipitation ; the zone of vegetation narrows, the scant pro- 

 tecting plant life disappears from the dunes, and they advance over the 

 edge of the loess belt and encroach also on the shrinking delta-plains. 

 With a period of renewal of precipitation vegetation resumes its former 

 area and the loess deposits expand over the dunes. 



The processes which we have reviewed have been operating with fluc- 

 tuating intensity since Tertiary time. 



The maximum of intensity existed probably as a consequence of the 

 Glacial period. 



Glacial epochs were accompanied by swollen rivers with broad flood- 

 plains, expansions of the seas with extensive marshes, and by great extent 

 of loess steppes. 



During interglacial epochs the conditions were reversed; and after 

 the last Glacial epoch there began the general trend toward the present 

 condition of aridity — a trend that was interrupted by oscillations, in 

 some of which the aridity may have exceeded that of today; a process 

 in which the seas, while responding to the oscillations, have in the main 

 shrunken gradually to the volumes compatible with the present equili- 

 brium between precipitation and evaporation. 



Parallel with this progress toward aridity, under the diminished pre- 

 cipitation and lessening to disappearance of the ameliorating climatic 

 reaction of the once expanded water areas, was the shrinkage of the loess 

 zones. The grassy steppes, which had once teemed with life and per- 

 mitted the distribution of ruminants and the horse across all Asia to 

 Europe, gradually became broken up into disconnected areas by the in- 

 creased intensity of desert conditions. 



The expanding deserts cut off the connection between the faunae of 

 southern Turkestan and Persia on the one hand and those of Europe on 

 the other, and allowed the evolutions of regional varieties; and there 

 must have been a similar reaction upon the distribution of man. 



After this, a continued progress toward extreme aridity advanced the 

 desert sea of sands till its dune waves, rolling ever nearer to the moun- 

 tain, completely submerged long stretches of the narrowed loess zone 

 between the now restricted deltas at the mouths of mountain streams. 



The teeming herds of ruminants and horses disappeared over vast areas, 

 and life was restricted to the mountains and to the borders of the few 

 remaining streams and the deltas. 



When this stage had been reached, in early prehistoric time and long 

 before the introduction of irrigation, the condition of southern Turk- 

 estan and northern Persia may be summed up as one of deserts relieved 

 only by oases on the deltas at the mouths of streams emerging from the 



