IRRIGATION SEDIMENTS. 35 
sediments were deposited over the old channel shown in shaft B, there has been 
no recurrence of dissection. Had irrigation not come to the rescue, the aggrading 
shore-line would have receded desertward, and the prolonging channels would 
have carried the sediments onward to form barren takyrs on the dune-covered 
plain. 
At present practically only the excess of water in the high floods carries fine 
sediments beyond the delta, for the water that is allowed to escape after the 
irrigating season is nearly clear. ‘Thus practically all the detritus is retained 
in the delta. The coarser material is rolled along in the ditches; the rest is dis- 
tributed over the fields, where it accumulates as a creamy white formation of even 
texture, of evenly mixed clay and very fine sand. The marked characteristic 
of this formation is an entire absence of stratification. This is doubtless due 
to growth of vegetation and the annual comminution of the soil through the 
processes of cultivation, for while the annual increment of sediment is only a small 
fraction of an inch, the yearly manipulation breaks up the ground to a depth 
of several inches. 
At present our only way of estimating rate of growth of irrigation sediments 
is by comparison with that of the accumulation of culture-strata. Both the city 
of Anau and the irrigation formation started on the natural surface of the delta; 
and, while in the city the culture-strata have grown to a height of 38 feet, the 
irrigation formation has risen on each side to a thickness of 15 feet, which would 
give a ratio of 1 of irrigation to nearly 2.5 of the loosely compacted culture-strata 
of the city of Anau. 
