290 PHYSIOGRAPHY OF CENTRAL-ASIAN DESERTS AND OASES. 
PRESENT OSCILLATION IN THE DEPOSITION ZONES. 
Four out of the five zones of deposition characteristic of a desert basin are 
found in Fergana, namely, glacial, alluvial, flying sands, and loess, while the 
lacustrian is lacking. An area of about 500 square miles of the central portion 
of the basin forms its nucleus of flying sands, while two other smaller nuclei of 
dunes are found in its western half—one just west of Khokand, the other crossed 
by the Syr where this basin narrows near Khojent at its western end. Most of 
the alluvial zone is barren gravel steppe and only its small areas of silt give rise 
to oases. Loess is nearly confined to its bordering uplands, a portion of which 
is composed of uplifted piedmont structures. The nuclei of flying sands are now 
expanding, steadily encroaching on the alluvial zone as its perennially flooded 
areas decrease, a fact evidenced by Russian and native records as well as our 
own observations. A considerable part of the loess zone, its lower areas, is now 
dead and suffering erosion, while living grass-covered areas have shrunken to 
pastures ranging above 4,000 to 6,000 feet. 
RECONSTRUCTION OF PAST EVENTS OF THE FERGANA BASIN. 
First cycle (Pliocene). 
Pliocene basin defined with high relief of border ranges eroding and building the plains 
of waste under desert conditions. Cycle closes with the low relief of Central 
Asia’s peneplain stage. 
Second cycle (Quaternary). 
High block uplifting of border ranges with gashing of their mature first-cycle topog- 
raphy, and corresponding marginal upbendings of the piedmonts constructed 
by the first cycle. Closes with well-widened mountain gorges partially alluviated. 
Third cycle (Quaternary). 
Similar to second, but of less magnitude. 
In the Alai Range, characterized by tilting of uplifted blocks raising that side next the 
plains. Nothing is yet known about it in the Tian Shan to the north. Its mar- 
ginal deformations of piedmonts built by the second cycle were on new lines 
farther out on the plains than those upheaved by the second cycle. 
Fourth cycle (Postglacial). 
Similar to the second and third, but of much less magnitude and still in the uplifting 
stage. 
Marginal deformations of third-cycle piedmonts on new lines still farther out on the 
plains. Climate in process of desiccation, flying sands expanding, alluvial activity 
shrinking, living loess areas shrinking to higher limits. 
This reconstruction of the past is of course purely tentative and subject to 
subdivision with cycles of less intensity or duration. 
