336 PHYSIOGRAPHY OF CENTRAL-ASIAN DESERTS AND OASES. 
natural sediments are nowhere over 10 or 15 feet thick. Their delicate laminations 
and extreme fineness of material differentially colored in thin bands of clay, for 
the most part buff and brown, are evidence of slow accumulation over a surface 
exposed to the oxidation of desert conditions. Although in a general way the 
growth of natural sediments came to an end with the beginning of irrigation and 
was superseded by that new kind of growth, we should expect to find layers similar 
to the natural at any horizon and interstratified with both culture and irrigation 
beds. From the time when one of the Murg-ab’s distributaries which fed Ghiaur 
Kala came under the control of man for irrigation, it ceased to be natural, 
1. e., became an artificial canal, and thenceforth any sediments deposited by its 
waters were other than natural. We have called those accumulating under the 
stratigraphically disturbing influences of cultivation irrigation sediments. Sedi- 
ments formed in choked-up canals, reservoirs, and abandoned fields may be termed 
canal sediments. The irregularities produced by occupation and irrigation of 
a plain, with such a gentle slope as the one with which we are dealing, inevitably 
result in the formation of extensive shallow depressions where, sooner or later, 
canal sediments accumulate. It is, then, canal sediments that have risen by the 
west wall and that form a 2-foot capping to the irrigation stratum cut by brick- 
yards southeast of the walls of Bairam Ali. 
Shafts 1, 11, and Iv penetrated characteristic dune-sand, so loose that shafts I 
and Iv had to be abandoned before the underlying loess was reached. But in 
shaft 11 we had just enough to give a key to the section and yet not enough to 
interfere with sinking, though the same mass attains a thickness of over 15 feet 
in the wall of the main excavation (lower digging), just to one side and a few feet 
above. Sand-dunes were evidently characteristic of the region before it was occu- 
pied and much of the city appears to overlie them. It is a significant fact that 
flying sands are found beneath both culture-strata and water-laid deposits, natural 
and artificial, and beneath it all is the loess. Every shaft that went deep enough 
found the great underlying mass of loess. Shaft 11 found it under dune-sand at 
—27 feet and sank 36 feet down into it, pure fine loess with vertical cleavage and 
calcareous concretions all the way down to where we stopped at water-level, 63 
feet below the surface. How much deeper it goes may be guessed, but there is 
no reason to doubt that it might be many hundred feet. 
THE STRATIGRAPHIC ORDER: (1) LOESS, (2) DUNE-SAND, (3) ALLUVIUM, EXPLAINED BY 
CLIMATIC CHANGE TO DRY, AND RECESSION OF THE DELTA. 
Now we are in position to correlate. The direct neighborhood of Ghiaur 
Kala had long been a loess steppe with topographical relief amounting to at least 
25 feet elevation between its summits and depressions. Asa result of some change 
in conditions it was invaded by flying sands, after which began the alluvial flooding 
and depositions over its lower porticns; and it was during this stage that the 
builders of Ghiaur Kala arrived to look upon a land of desert dunes and playas, 
with here and there a remnant of the old half-drowned loess topography. Upon 
one of these remnant loess masses rising about 16 feet above the flood-plain of a dis- 
