644 R. PUMPELLY EVOLUTION OF OASES AND CIVILIZATIONS 



face rises slowly during the centuries to form great thicknesses of the 

 soil we call loess. 



Look back again over the region ; while the sand from which was sepa- 

 rated the dust you have just seen deposited to form loess lags still scores 

 of miles behind in its advance, you see the grassy plains bordered by a 

 sea of high and older sand dunes. They, too, have been arrested in their 

 overwhelming progress by the slight growth of grasses and plants that 

 are compatible with a soil of sand, under the slight precipitation near 

 the border zone. Both the loess and the dunes grow continually in 

 height. 



You have seen a cycle of geological activity quite different from that 

 which takes place on the periphery of a continent where the silts are 

 distributed by ocean currents over great submarine areas. 



Here, on the contrary, the waste from the degrading mountains, which 

 was spread by rivers over the plains, is returned by the winds to pile up 

 on the piedmont zone, and this is obviously true not only of the solids, 

 but of the soluble alkaline and earthy salts as well. 



All this conforms strictly to Eichthofen's theory, that loess is a product 

 of deflation of desert surfaces, wind-borne till it found protection on the 

 grass-covered zone. Here, however, we see that water intervened as an 

 earlier transporting agent, and that evaporation, on the plains, restored 

 to the fine silts the salts that had been leached out. That loess may 

 form without the intervention of water we have seen in the extensive 

 deflation of rocks on the high deserts of the Pamirs. 



Let us return to your panorama; it is still that of many thousand 

 years ago, and the grassy steppes across all central Asia teem with herds 

 of wild ruminants and horses and other animals that during early 

 glacial and interglacial time were common to the Eurasian continent. 



I will ask you to look, at the same time, toward the edge of the plains. 

 At short intervals you will see streams emerging from the mountains 

 through canyons onto the plain, where they spread out evenly over large 

 fan-shaped deltas that slope radially outward from the apex at the 

 canyon mouth. These are the delta oases, of which I shall have more 

 to say. 



Casting your eye along the southern border of the plains, from the 

 Caspian sea eastward you see grassy loess-plains fringing the southern 

 mountains and filling out the great embayments between the spurs of 

 the Tienshan ranges in the east. But everywhere both these plains and 

 the deltas are hemmed in by the sea of dunes. 



During your foreshortened time scale your present glance sees the 

 effects of later climatic oscillations. It is perhaps a period of diminish- 



