CONCLUSIONS AS TO PLEISTOCENE CLIMATIC STROPHE 383 



an inquiry into the phenomena of arid regions. In such regions the 

 most obvious features due to change of climate are lacustrine strands. 

 These are well marked in the three basins of Seyistan, Lop, and Turfan, 

 and also in other basins of central Asia not here discussed. They do not 

 differ essentially from the classic examples of lakes Bonneville and La- 

 hontan. Where best developed, in the Lop-Nor region, they number five 

 main strands and a sixth minor one. BetAveen the glaciers of the upper 

 parts of the streams and the lakes at the other extremity, the river valleys 

 of central Asia are characterized by terraces, apparently of climatic 

 origin, which agree in number and size with the moraines at one end and 

 the strands at the other. The terraces are of high importance because, 

 if they have been rightly interpreted, they preserve a record of the 

 changes of the Pleistocene strophe not only in central Asia, but in other 

 arid regions, such as northern India, the Deccan, and the basin region of 

 the United States. Both terraces and strands, important as they are, 

 suffer from the same limitations as do moraines. They afford an in- 

 complete record, because the work of less severe epochs is liable to efface- 

 ment during later epochs of greater severity. 



The records of climatic change preserved in the alternating lacustrine 

 and subaerial deposits of the bottoms of inclosed basins suffer from no 

 such limitations; they are complete; hence their great importance in the 

 few places where they are exposed. A study of the three basins of Seyis- 

 tan, Lop, and Turfan shows that the depositional records of a climatic 

 strophe in the bottoms of basins comprise at least two main types of 

 alternating strata. The first consists on the one hand of greenish or 

 light colored layers of lacustrine origin, and on the other of reddish sub- 

 aerial layers, the color of which indicates that they were deposited under 

 conditions of great aridity. The second consists of lacustrine layers 

 alternating with subaerial vegetal layers, which indicate the presence of 

 swamps at times when the lake contracted.* Both types of strata show 

 that in central Asia the Glacial period was a time of oscillations between 

 epochs of abundant water supply and those of aridity, during which the 

 water supply was as small or smaller than at present. This would nat- 

 urally be expected from what has been learned of the Glacial period in 

 other parts of the world. A more unexpected result is found in the evi- 

 dence of the three basins as to the length and complexity of the Pleisto- 

 cene strophe. It appears that, in central Asia at least, the strophe con- 

 sisted of a large number of cycles, part of which preceded those known 

 elsewhere as the Glacial period. Apparently the entire strophe consisted 



* A third type of formation indicative of climatic change ought to be mentioned, 

 namely, alternating strata of gravel and of finer materials. It has been so little studied 

 that it is omitted above. 



XXXIII — Bull. Geol. See. Am., Vol. 18, 1906 



