THE PLEISTOCENE OR GLACIAL PERIOD. 465 



the two stages of expansion; and the youngest were made at the time 

 of the last expansion. Oolite was deposited at some stages of the 

 lake's history, and is now making about Pyramid Lake. In some parts 

 of the Lahontan basin there are deposits of salt, and salt is also derived 

 from brine wells. 



Subsequent to the last stage of expansion, the waters appear to 

 have been completely dried up. The period of maximum desiccation 

 is thought to have been no more than 300 years ago. Since then the 

 humidity of the region has so far increased as to develop small lakes 

 in the deeper parts of the former basin. 



All lines of evidence point to the shortness of the time since Lakes 

 Bonneville and Lahontan existed. The embankments of sediment 



BiB 



Fig. 546. — Faulting in sediments of Lake Lahontan, Walker River Canyon, New 

 (Russell, U. S. Geol. Surv.) 



about the old borders of the lake seem to be almost as perfect as when 

 formed, even the valleys which cross the terraces being small. It is 

 to be remembered, however, that the region is arid and its sediments 

 porous, conditions which do not favor the ready destruction, or even 

 the ready disfiguration, of terraces, deltas, etc. Russell infers that 

 the desiccation of the lake was probably accomplished centuries, but 

 probably not many thousands of years ago. 



Recent as the closing stages of Lake Lahontan's history were, 

 there have been considerable diastrophic changes in the region since, 

 for faults affect the lacustrine sediments at various points (Fig. 546). 

 Some of these faults have been traced more than 100 miles, and the 

 throw of some of them is not less than 100 feet, though the amount is 

 usually less. The recent fault movements seem to have been mainly 

 along the lines of earlier faulting (Fig. 547). It is worthy of note that 

 the numerous hot springs of the region are mostly along the lines of 

 recent faulting. This has led to the inference that the friction of 

 faulting was the source of the heat, but this is clearly not the only 

 interpretation possible. 



