224 THE BASIN OF EASTERN PERSIA AND SISTAN. 
meastire to the presence of mountain barriers which shut out the moisture-laden 
air of the oceans from the basins which they inclose. Thus the fonnation of basins 
tends to produce a dn>- climate, and the dr}- climate tends to preser\'e the basins and 
at the same time to produce a peculiar topography. 
It is not simply with aridil)- of climate that we have to deal in Eastern Persia. 
During Quaternary times there appear to have been changes of climate, and as some 
of the changes took place ver)- recently, probably since the occupation of the countrj' 
by man, their careful study is of great importance for the purpose of our expedition 
The chief evidences of climatic changes take the form of numerous lacustrine and 
fluviatile terraces. The former, like the terraces of Lake Bonneville, are due to 
changes in the water-level of lakes or playas, while the flu\'iatile terraces appear 
to have been fonned where one type of climate caused the deposition of gravel, and 
another t)'pe, probably either drier or wanner, caused this to be channeled. The 
physiographic effect of changes of climate is so important that its exemplification 
in Persia demands the most careful consideration. The uniformity of the terrace 
phenomena throughout the semi-arid countries of the western half of Asia seems 
to be explicable only on the theory of a succession of epochs of changing climate 
corresponding to the glacial epochs of more northern countries. 
The lake and district of Sistan afford unusually clear evidence as to the sub- 
division of Quaternary' time. During the latter part of the Quaternary era volcanoes 
broke out within the area of the lake, and in the course of their eruptions large 
portions of the lake bottom were uplifted and covered in part with caps of lava- 
Subsequent erosion has produced cliffs from 50 to 600 feet high, which expose large 
])ortions of the ancient lake deposits. The histor}' of the Quaternan,- era and of the 
forms assumed in Persia by the period which corresponds to the glacial period of 
other lands is here laid bare without the concealment of earlier phases and without 
inidue emphasis on later events. 
The record of the climatic changes of the QuaternarA^ era is almost ever^^'here 
incomplete, whether pre.ser\'ed in moraines, in terraces, or in aqueous deposits. One 
fonnation is placed upon another, and unless each successive epoch happens to be 
less severe than its predecessor, the traces of earlier epochs are almost sure to be 
effaced. The records of climatic change are preser\-ed most perfectly in the bottoms 
of shallow lakes without outlets, where a diminution in rainfall or an increase in 
evaporation produces a great diminution in the size of the body of water and con- 
sequently in the character of the .sediments deposited. Naturally the bottoms of 
such lakes are of little use to the geologist, because of his inability to stud)- them. 
Hence the importance of Sistan, where so large a part of the record is exposed. It 
affords a key which may serve to inilock the history- of the neighboring Caspian 
basin and of still larger regions. 
The deposits uplifted at the time of the Sistan volcanoes and exposed to view 
by the erosion of the lake consist of layers of reddish clayey silt varied with bands 
of sand and gravel on the one hand, systematically alternating with remarkably 
uniform unbroken layers of hard, greenish claj- on the other. The reddish lajers 
contain lateral unconfonnities, discontinuous layers of coarser material and rain- 
drop prints, which indicate that they are of subaerial origin and were laid down by 
