244 PHYSIOGRAPHY OF CENTRAL-ASIAN DESERTS AND OASES. 
Gobi basin. ‘There, as in Fergana and along the base of the Kopet Dagh, these 
uptilted piedmont formations are a magnificent corroboration of the idea of dis- 
placement, as well as of Suess’s theory of the encroachment of mountains on plains. 
A desert basin, then, is organically divided between mountains and plains, 
but this is only the beginning of our classification. Functionally, the plains are 
a vast geologic mill in which the material received is differentially assorted into 
layers of fine and coarse alluvium, whereof the surface is further sifted into loess 
and flying sands ere it comes to rest under succeeding layers; and this mill is 
worked by wind and water. It is a plain whereon the muddy floods of spring 
and fall give rise to momentary shoals of water spread over many scores of square 
miles, welcome lakes that vanish under the burning sun, to leave mirage and wind- 
swept barrenness of sandstorms and yellow days. But of this water some remains 
more permanently wherever the supply is in excess of evaporation. ‘The life of 
a desert lake or sea is, according to circumstances, anything from a day to a cycle 
of geologic time; anything from the momentary existence of a thin watery sheet 
far out among the dunes to the history of an Aral Sea. 
Thus arise four marked subdivisions of deposition—alluvial, lacustrian, flying 
sands, and loess—two of water and two of wind. Lastly, but perhaps most 
important in records of Quaternary change is the fifth subdivision of deposition, 
glacial ‘‘till.’”’ A desert basin is thus divided into areas of erosion and deposition, 
mountains and plains; the plains are divided into four zones—alluvial, lacustrian, 
flying sands, and loess—and the loess zone includes a portion of the mountains, 
while a fifth subdivision of deposition, glacial, is found on the higher mountains, 
THE THREE AGENCIES OF EROSION AND FIVE DEPOSITION ZONES. 
There are three agencies of erosion and transportation, ice, water, and wind, 
and five deposition zones, glacial, alluvial, lacustrian, flying sands, and loess 
arising therefrom. 
Rising among the glaciers and snow-clad peaks of the inclosing mountains, 
small and large silt-laden streams discharge upon the plains. Flood gives way to 
the drought of a burning sun that stirs the atmosphere into vast cyclonic storms 
and spiral dust-whorls—tall, shadowy forms that come and go in ever-changing 
shape, born out of the horizon to wander a while and vanish. By these atmos- 
pheric disturbances the surface materials are consumed and sifted over, digested 
into drifting sands and far-blown dust. Most of the dust is borne far away to rest 
as loess in the grass of high valleys and plateaus flanking the peripheral mountains, 
for it can not survive a wind on barren surface; but sand moves slowly to and fro 
in the shifting winds, and only that which gets beyond the ultimate shores of 
alluvial activity accumulates to form the larger masses we call dunes. Probably 
the most important source of this sand lies in the more or less impure sandspits 
that are invariably to be found after flood along distributary channels of silting 
streams. Any shifting aggregations that have not found their way onto an area 
more or less permanently free from alluvial activity must suffer rearrangement 
by the next flood, but in the vast nuclei of flying sands that characterize the desert 
plains we have ample proof of the large scale at which the wind has been successful. 
