THE PLEISTOCENE OR GLACIAL PERIOD 909 



lations of drift are in the form of lateral moraines which in some 

 cases are nearly or quite 1,000 feet high. 1 Under the conditions of 

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

 debris was carried bej^ond the ice by the water emanating from it, 

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

 Glacial cirques, the result of a peculiar phase of glacier erosion, are 

 well developed in many of the glaciated valleys, as for example, in 

 the Uinta Mountains. 2 



The characteristics of mountain valleys which were occupied 

 by considerable glaciers, are essentially constant. They include 

 (1) well developed cirques at the heads (PL XVII); (2) the upper 

 parts of the valleys were so thoroughly cleaned out by the ice that 



Fig. 599. Fault scarps in the moraine at the mouth of the Little Cotton- 

 wood Canyon, Wasatch Mountains. (Gilbert, U. S. Geol. Surv.) 



little loose debris, except that due to post-glacial weathering, re- 

 mains; (3) numerous tributary valleys are hanging (Fig. 209), and 

 their waters form cataracts; (4) at and near the limits of the ice, 

 at stages when its end or edges remained nearly constant in position 

 for a time, there are heavy accumulations of drift, lateral moraines 

 being as a rule more conspicuous than terminal; (5) the valleys 

 contain lakes (PI. XVII), some of which occupy rock basins, and 

 some basins produced by drift dams; and (6) valley trains or out- 

 wash plains below the moraines. The partial removal of these 

 deposits has developed terraces (Fig. 163). 



Glacial lake deposits. By obstructing valleys, the mountain 

 glaciers of the west gave rise to numerous temporary lakes in which 

 lacustrine sediments were laid down. The extent of such lakes in 

 the west and northwest has not been determined, but where glacia- 



1 This means that the drift is 1,000 feet deep. The crests of the lateral 

 moraines are locally 2,500 feet above the valley bottoms. See Cloud Peak, 

 Wyo., folio, U. S. Geol. Surv. and Professional Paper 62 (Atwood). 



2 See Hayden Peak and Gilbert Peak, Utah, topographic sheets of the U. 

 S. Geol. Surv., for fine examples of large cirques. 



