472 GEOLOGY. 



that the last glaciation of the west can hardly have preceded the Wis- 

 consin glacial epoch of the interior. Nevertheless there has been 

 much post-glacial weathering, especially that resulting from the expan- 

 sion and contraction due to changes in temperature. In favorable 

 localities, this has resulted in the development of enormous bodies 

 of talus, some of which are said to be 1000 feet in thickness. 1 Such 

 accumulations arc most extensive in the Sierras. 



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 the geographic and climatic conditions. The mountain 

 ranges of the basin region are separated by broad valleys. 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 surround- 

 ing 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 1000 feet in height, are found. 2 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. Where the adjacent mountains are of limestone, 

 the detritus against their bases is often firmly cemented into breccia 

 by lime carbonate. The geographic conditions in the basin region 

 are such as to cause most of the coarser products of erosion from the 

 mountain to be deposited on the lowlands about them. If the Quater- 

 nary talus and alluvial deposits were sharply separable from those 

 of late Tertiary age, they would afford a rough measure of the Quater- 

 nary erosion in the mountains . 



As the glacial deposits increase in importance to the north, talus 

 and other subaerial accumulations become less conspicuous, and are 

 of much less importance in Montana, Idaho, and Washington, than 

 in the more arid regions farther south. 



Talus accumulations take on various forms, as shown in Figs. 554 

 to 556. Fig. 554 shows talus in its normal form. Fig. 555 shows a 

 type of accumulation not uncommon in the western mountains. In 

 some cases at least this disposition of the talus appears to be due to 



1 King, op, cit., p. 472. 2 King, op. cit 



