910 GEOLOGY 



tion was extensive, derangement of the drainage was common, 

 and deposits of glacio-lacustrine clay, hundreds of feet deep, an* 

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

 valleys now drained, they have been partly removed, 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 occurs in greatest 

 quantity where canyons issue from the mountains, and in such 

 situations huge fans of bowlders, sometimes 1,000 feet in height, 

 are found. 1 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 of m.uch less 

 importance 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. 62), 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 tula arc 

 now making (Fig. 175). Considerable deposits of a similar nature 

 antedate the present by a notable interval of time, but probably 

 fall within the limits of the Quaternary period. 



Marine deposits. At some points along the western coa 

 the United States marine deposits reach inland some distance from 



1 King, Oeol. Surv. of 40th Parallel, Vol. I. 



