1 62 . GEOLOGICAL SURVEY OF THE TERRITORIES. 



ness. In section No. 19 of Tertiary strata we have seen that there are 

 over 7,000 feet of beds. A large part of their ingredients was doubtless 

 derived from the Sawatch range. Over the mass of the Elk Mountains, 

 the Cretaceous, with underlying strata, have been removed, to a great 

 extent, leaving only on the edges remnants of the Cretaceous. On the 

 western flank of the Sawatch we see no Cretaceous. This will give us 

 some idea*of the enormous denudation that has been effected. A large 

 portion of this denudation was doubtless the result of glacial action. 



The erosion going on at the present time is by no means inconsider- 

 able in amount. The high water during the spring months carries 

 down a vast quantity of material. During the rainy seasons the rain 

 comes in frequent showers, which, although of short duration, are very 

 violent, and wash down immense quantities of material, cutting deep 

 gullies into the bluffs. The softness of the beds on Eagle River, on 

 Grand Eiver below Roaring Fork, and on the North Fork of the Gun- 

 nison, renders them very susceptible to aqueous influences. In the 

 mountains, the alternate freezing and thawing of the snow-banks, in the 

 spring and summer, has considerable influence on the erosion. The 

 alteration in the level of the streams is often very perceptible. 



Another agent of erosion has been pointed out by G. K. Gilbert, in a 

 paper read before the American Association for the Advancement of 

 Science, in 1874, viz, that caused by the action of sand carried down 

 the streams on the rocks through which they flow. The erosion effected 

 in this way is very considerable, especially where the beds of the streams 

 are in solid rock. Our district was so broken by hills that the action 

 of wind in eroding the strata is not so noticeable as in more level coun- 

 tries. 





