KllHTZE, GEOLOOY OF WASATCH MOUNTAINS, UTAH 91 



and sandstones, while in the lower and steeper part of the canyon they 

 pass into hard quartzites and slates. 



It thus seems to be a fact that the structure and hardness of the rocks 

 in the upper part of the canyon have had a somewhat greater influence 

 on the course of the stream than in the lower part. Little Cottonwood 

 Canyon is developed for the most part in granite of a very hard and 

 homogeneous character. The course of the canyon is parallel to Big 

 Cottonwood, where both structure and heterogeneous rocks enter into the 

 problem. It is apparent that there must be some other cause operative 

 to produce the correspondence. The chief determining factor seems to 

 have been the original form of the block mountain. The western conse- 

 quent streams on the steep fault face developed their channels transverse 

 to the main north-south trend of the block, their direction being deter- 

 mined by the slope primarily. If the block was rapidly uplifted, the high 

 gradient of the streams would be quite sufficient to cause them to cut 

 back independent of the structure and kinds of rock. The direction of 

 back-cutting would be at right angles to the front of the block, and as 

 this was somewhat irregular, being curved in places, the stream courses 

 should show some irregularity in direction. This indeed is the case. 

 Where the fault face forms a great curve, as it does southeast of Salt Lake 

 City, the canyons show a marked tendency to take off in the direction of 

 the extended radii of the arc, as should be expected. 



GLACIATION 



After the Wasatch block mountain had been maturely dissected by 

 stream action as briefly outlined above, Alpine glaciation set in during 

 the Pleistocene period. Many of the deep Y-shaped gorges were hollowed 

 out into broad U-shaped valleys of striking outline. The best known ex- 

 ample is Little Cottonwood, but there are many others in the upper parts 

 of the large canyons. The upper half of Big Cottonwood is a deep U- 

 trough with many hanging valleys on both sides. The heads of the can- 

 yons were widened into broad catchment basins with steepened sides. 

 The divides were greatly sharpened in many places. Altogether, the 

 topography was modified to a considerable extent in the central Wasatch, 

 especially at the higher elevations near the heads of the canyons.^ Nu- 

 merous lakes due to glacial damming and the plucking action of the ice 

 by which rock basins of considerable depth were formed are to be found 

 at the heads of the larger canyons. Good examples of rocJie moutonnees, 



* For a map showing the location of the principal glaciers and their catchment basins, 

 as well as a brief account of the glaciation in the Wasatch, see Atwood : U. S. Geol. 

 Surv. Prof. Pap. No. 61. 



