88 ANNALS NEW YORK ACADEMY OF SCIENCES 



and the surface was reduced to an aspect of low relief. Then followed a 

 period marked by profound faulting, the lines of movement being prin- 

 cipally in a north-south direction, but with many cross fractures, which 

 resulted in the formation of great fault-block mountains. These were 

 characterized by relatively simple external features but with complex 

 internal structure. The most easterly, and one of the most continuous 

 of these fault-block masses, is the present Wasatch range. 



When newly formed, the Wasatch block had a steep western face and a 

 long gentle eastern back slope. It was greatly elongated in a north- 

 south direction, extending from central Utah northward for almost 200 

 miles. The width as measured from its fault face on the west to its 

 eastern border was about 25 miles. Its height was mainly due to vertical 

 displacement along the great fracture line on the west. This dimension 

 was no doubt cumulative and due to periodic uplift, the aggregate throw 

 probably reaching 10,000 feet. The line of greatest elevation or crest of 

 the block was near the western margin. 



DISSECTION AND DRAINAGE 



The dissection of such a block must have been initiated by the conse- 

 quent streams which flowed down the two unequal slopes to the east and 

 west. The valleys developed by these opposed streams would thus ba 

 transverse to the principal direction of the range, and when fully devel- 

 oped would divide it into a series of roughly parallel east-west ridges on 

 each slope, leading from the main divide to the two margins of the block 

 mountain. The unequal declivity of the two sets of streams would in 

 time cause a migration of the divide toward the center line of the block, 

 if the structure and materials were not essentially different and the base 

 levels were at the same elevation on both sides. If the base level on the 

 east were higher than the one on the west, the divide would come to rest 

 nearer the eastern border, and the valleys and ridges west of it would be 

 longest and most prominent. Some of the most powerful streams on the 

 west slope might even cut entirely through the divide and send out lateral . 

 subsequent tributaries that would capture the east flowing consequents 

 and lead them westward into the Salt Lake Basin. When once estab- 

 lished, these master streams would continue to push eastward into the 

 region beyond the Wasatch, gradually acquiring more and more drainage 

 territory. 



In the light of these theoretical considerations, we may examine the 

 present maturely dissected Wasatch block mountain for some of the 

 larger features due to its original form and subsequent dissection. 



