66o THE PLEISTOCENE PERIOD 



contain lakes (PI. XIII), 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. 124). 



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- 

 tion was extensive, derangement of the drainage was common, and 

 deposits of glacio-lacustrine clay, hundreds of feet deep, are known 

 at some points. Where such deposits were made in narrow valleys 

 now drained, they have been removed in part, and their remnants 

 constitute terraces. 



Alluvial and talus deposits. In the basin region of Utah and 

 Nevada there are exceptional deposits of detritus, the accumulation 

 of which was favored by topography and climate. The mountain 

 ranges of the basin region are separated by broad depressions. 

 From the steep slopes, detritus is carried down both by descending 

 torrents and by gravity, and while it is largely deposited at and 

 against the bases of the mountains, some of it is spread widely over 

 the surrounding plains. This debris is mainly unstratified, or poorly 

 stratified, and some of it is very coarse. It appears in greatest 

 quantity where canyons issue from the mountains, and in such situa- 

 tions there are huge fans of bowlders, some of them 1,000 feet in 

 height. The torrents were able to carry this coarse material so 

 long as they were confined within the canyons, but with the change 

 of gradient below, the water gave up its load. As the glacial de- 

 posits increase in importance to the north, talus and other sub- 

 aerial accumulations become less conspicuous, and are much less 

 considerable in Montana, Idaho, and Washington than in the more 

 arid and unglaciated regions farther south. 



Eolian deposits. The wind is an important agent of erosion 

 and deposition in the west. Its erosive work is shown in the pecu- 

 liar carving which affects the cliffs and projections of rock at many 

 points (Fig. 14), and its depositional work by the dunes, which 

 are not rare. The erosive work of the wind here is far greater than 

 is commonly appreciated by those unfamiliar with arid regions. 



Deposition from solution. About many springs, as in the Yel- 

 lowstone Park, deposits of siliceous sinter and calcareous tufa are 

 now making (Fig. 31). Considerable deposits of a similar nature 



