THE PLEISTOCENE OR GLACIAL PERIOD. 467 



Mono Lake. — A lake which occupied a part of Mono Valley, Cali- 

 fornia, 1 had a similar history. The two stages of high water here are 

 associated with two separate advances of the ice. Glaciers descended 

 into its basin below the level subsequently reached by the water. 

 As in the case of the larger lakes farther east, there has been faulting, 

 deformation of the beach lines, and volcanic action in the basin of 

 the lake, since the last retreat of the ice. Mono Lake seems to have 

 been without life throughout most of its history. 



There were numerous other Pleistocene lakes in the Basin and moun- 

 tain regions, but their histories have not been worked out in detail. 



Glacial effects. — The extent of glaciation in the western mountains 

 has been outlined in the early part of this chapter. Throughout the 

 area glaciated, there is evidence that the erosive work of the ice was 

 great. This is shown both by the extensive deposits of glacial and 

 fluvio-glacial origin, and by the forms of the valleys occupied by the 

 ice. At the east base of the Park Range in Colorado, for example, 

 there are said to be terminal moraines 1000 feet high. 2 In the Uinta 

 Mountains, the terminal moraines are much less massive, but lateral 

 moraines 1000 feet high 3 are found. Under the conditions of active 

 drainage which existed in the mountains, much of the glacial debris 

 was carried beyond the ice by the water emanating from it, and deposited 

 in the valleys and " parks," or on the plains below. Xowhere in the 

 world where accurate topographic maps have been made, arc glacial 

 cirques, the result of a peculiar phase of glacier erosion, better developed 

 than in these mountains. 4 



The characteristics of the mountain valleys which were occupied 

 by considerable glaciers, arc essentially constant. They include (1) well 

 developed cirques at the heads (Fig. 5-^8 and PI. XIX, Vol. I); (2) the 

 upper parts of the valleys, often for some distance below the cirques, 

 are so thoroughly cleaned out, that little loose debris, except that 

 due to post-glacial weathering, remains; (3) numerous tributary 

 valleys are hanging (Fig. 2G2, p. 290, Vol. I), and their waters form 

 cataracts (Fig. 263, p. 291, Vol. I); (4) at and near the limits of the 



1 The Pleistocene History of Mono Valley, Russell, 8th Ann. Rcpt. U. S. Geo!. Surv. 

 ? King, op. cit., p. 468. 



3 This means that the drift is 1000 feet deep. The crests of the lateral moraines 

 are locally 2500 feet above the valley bottoms. 



4 See Hayden Peak and Gilbert Peak, Utah, topographic sheets of the U. S. Geol. 

 Surv., for fine examples of large cirques. 



