DESERTS. 295 
ward over the Hunger Steppe and Fergana plains, 7. e., widening in proportion 
to the height of mountains drained. This alluvial zone, furthermore, extends 
into the great Sand, where it is penetrated by the rivers Tedjen, Murg-ab, and 
Zerafshan, and where it is divided by the rivers Syr and Amu crossing to the Aral 
Sea. Now, it is a fact of significance that all five of these large rivers, as well as 
many smaller ones that still reach, or have recently reached, well out onto the 
plains, have cut channels from ro to 100 feet or more in depth to where they 
debouch over deltas. It is, moreover, characteristic of these channels that they 
vary in depth in such a way as to indicate a varied warping of the plains. And 
though most of them are still occupied by streams, there are many instances of 
channels now always dry, but so recently abandoned by the streams now ending 
many miles above in a shrunken condition that ground-water still survives, obtain- 
able in shallow wells of the nomads. On our large-scale Russian maps there are 
remarkable fragments of such channels so far removed from present alluviation 
that it is difficult, sometimes impossible, to say what river they belonged to. 
Others appear to have been the work of distributaries cutting into the plains 
they had once overflowed. Where distributaries have been thus incised, we have 
definite proof of crustal movement. Our most striking examples of distributaries 
cut into a warped plain are afforded by the Zerafshan, while of those cut into the 
zone of uptilted piedmonts we find most remarkable examples along the southern 
border of the Fergana plains. 
The vast alluvial zone of this basin was built by its rivers when they wan- 
dered freely. Now most of them are relatively fixed. That the Turkoman 
Trough was at one time the Amu’s flood-plain, when that river flowed to the 
Caspian, building the immense deltas characterizing the coast south of Krasno- 
vodsk, seems more than likely. That would be postglacial. Then it and doubt- 
less most of the large rivers were unconfined and spread a large portion of their 
load on the plains, whereas silt of the Amu and Syr of to-day is mostly in transit 
to the Aral. This period of free-shifting rivers with unconstrained alluviation 
was followed by warping. Here we must remember the postglacial uplift of 
mountains, the peripheral uplift of our fourth erosion cycle. The warping of 
plains, uptilting of their margins, and uplift of their border ranges fall logically 
together into one cycle of a basin’s differential movements. As a confirmation 
of this idea we have the corresponding increase of aridity, shrinkage of sea-water 
area, contraction of streams, shrinkage of living loess, and expansion of flying 
sands, and, finally, depopulation of oases. 
RECENT CHANGES IN THE COURSE OF THE OXUS (AMU DARYA). 
The archeologic and historic period of this basin is treated under “ Physi- 
ography of Oases,’’ chapter xv, this report, but there has been so much dis- 
cussion about historic changes of river courses, especially of the Oxus, that a 
physiography of the basin must take up the problem. Elisée Reclus states: 
The great changes that have taken place in the course of the Oxus within the historic period are amongst 
the most remarkable physiographic phenomena, comparable in recent times only to the periodic displace- 
ments of the Hoang Ho. . . . . In the days of Strabo the Oxus, ‘‘largest of all Asiatic rivers except 
