THE QUATERNARY ERA IN SISTAN. 287 
exception of two on the Hehnund River, each section contains three distinct parts. 
At the top, lying imconforniably on the other strata, are beds of orravel from one 
to a hundred or more feet thick. Below these are numerous alternations between 
thick pinkish strata and thinner strata of a greenish or whitish color. 
The pink beds consist largely of clays and ver}- fine silts, but often pass into 
layers of fine brown sand. Viewed as a whole, the pink laj'ers are \ery continuous, 
and preserve the same character for mile after mile. In detail, however, they vary 
considerably, even in short distances. For instance, a layer of clay is often inter- 
rupted by a band of fine sand which continues a few hundred feet and then dies 
out. More rarely a la\-er of grit or gravel occurs and, rarest of all, a distinct fossil 
stream-bed of gravel is exposed. Again, in certain places slight unconformities are 
discernible, as though a brief period of erosion had taken place between the deposi- 
tion of one layer and the next. Among the more sand)- lajers there are further 
evidences of exposure to the air. In one place, for instance, the sand shows ripple- 
marks, worm-casts, and rain-drop prints. Lastly, the layers of this formation are 
ever^'where of a reddish tint, varying from pink to brown. The only exception is 
found in some of the sandier, more quartzose layers, which are gray for a few inches. 
Everywhere the materials seem to have been exposed to oxidization for a considerable 
period. They bear the marks of having been deposited subaerially by widely 
spreading floods or in temporary playas. 
The white or, more exactly, the greenish clays, on the other hand, present a 
ver>' different appearance. On the edges they are mixed with fine sand or are more 
or less banded. Occasional!)' a purple layer occurs, or a band of yellow clay, in 
which are what appear to be fossil leaves and reeds. The main mass of each stratum, 
however, consists of solid, unbroken layers of pure clay, unifonn in texture and 
color, and showing none of the slight variations which characterize the pink beds. 
The color indicates that the materials were brought rapidly from their place of 
origin in the mountains and were not long exposed to o.xidization on the way. The 
green strata as a whole show no sign of subaerial origin, and appear to be t)pically 
lacustrine. 
The pink beds and the green beds differ from one another chiefly in manner of 
deposition. The material of the clayey portions of the pink is identical witli the 
green, except that it is more weathered, and it is reasonably certain that they were 
deri\ed from the same source. The discontinuous, highly- weathered pink layers, 
however, appear to have been deposited subaerially after long exposure to the atmos- 
phere, while the unifonn, slightl)--weathered green layers appear to have been 
deposited subaqueously after a relatively short exposure to the atmosphere. Such 
alternations of subaerial and subaqueous conditions indicate that the lake of Sistan 
has been subjected to changes whereb)- a portion of its bed has been altematel)- 
exposed as dr)' land and submerged under water. The duration of each epoch of 
submergence or exposure must hav-e been considerable, for the accumulation of from 
5 to 20 feet of the finest clay, or of a greater thickness of a clayey silt and sand, is 
a slow process mea.sured in our small unit of years. This is especially true if in 
former times deposition was as slow as at present. Mr. Tate told me that during 
