CHANGE-PRODUCING AGENCIES 659 



brown loam, which indicates clearly that loess-dust enters into it as an 

 important constituent. Of the upper 60 feet, 50 consist of more or less 

 coarse material in which the brown loam is absent. 



The conditions that permitted the forming of these great thicknesses 

 of brown loam were apparently those belonging with a greater amount 

 of both general and local precipitation. They 'presuppose, I think, a 

 degree of moisture that does not now obtain, under the influence of 

 which there was a perennial growth of grass sufficient to allow the 

 growth of intimately mixed alluvial silts and loess-dust. 



Of the upper 60 feet of this column, 50 feet consist of more or less 

 coarse material without brown loam, and I imagine that the top of the 

 brown loam at — 60 feet in the well is proximately contemporaneous with 

 the similar material under the north and south kurgans, and that its 

 greater depth may roughly correspond to the depth to which degradation 

 extended before the refilling of the valley occurred, during which the 

 north kurgan was started. 



, The absence of the brown loess-dust constituent, both from the upper 

 60 feet in the well and in the sediments deposited at Anau after the 

 starting of the north kurgan, points, I think, to a diminished precipita- 

 tion over the piedmont zone; that is, diminished sufficiently to cause a 

 deficiency in the growth of grass required to retain the loess-dust. 



When we compare, further, the upper 60 feet in the well with the 

 whole of the column below, we see that there is evidence of a great 

 change from a long-continued different condition ; and when we consider 

 together the apparent decrease in vegetation indicated by the absence 

 of the loess constituent, and the evidences, both geographical and arche- 

 ological, of regional desiccation, the change would seem clearly to have 

 been toward aridity. The successive degradation and rebuildings re- 

 corded in our shafts show that this period was one of fluctuating 

 climate — a time in which the periods of greater precipitation affected 

 the mountain regions without causing local rainfall, after winter, on the 

 zone of deposition. 



The time needed for the accumulation of the observed 2,300 feet of 

 sediments in the Askabad well can be estimated only in geological 

 chronology. It doubtless extends well back in the Pleistocene period, 

 and it is not unlikely that the conditions shown in frequent recurrences 

 of coarse cobble-beds between the depths of 500 and 900 feet mark the 

 last great glacial advance. 



Looking on the loess-forming condition shown below — 60 feet as 

 typical of the piedmont plains of southern Turkestan generally and 



