PEEFATOEY NOTE. ix 



River Mountains on the north; in the Park Mountains on the east, and a 

 number of tributaries come from the west. In their courses through tlie 

 plateaus they run in canons. These caiions are profound gorges corraded 

 by the streams themselves. The "country rock" of the region is composed 

 of sedimentary beds, nearly horizontal, as already stated. The region is 

 also excessively arid, but the mountains that stand on the rim of the basin 

 precipitate a large proportion of moisture, and in this manner streams 

 of comparatively large volume head in the mountains, run through the 

 plateaus and descend rapidly to the level of the sea, while the country 

 through which they pass is very meagerly supplied with moisture. Under 

 these conditions the profound gorges have been cut, as the process of canon 

 cutting is more rapid than the lateral degradation of the country. In this 

 manner every river runs in a deep gorge, and these canons further serve to 

 divide the region into plateaus. 



The division is completed by lines of cliffs. These cliffs are bold escarp- 

 ments hundreds and thousands of feet in altitude — grand steps by which 

 the region is terraced. As the rivers corrade their channels more rapidly 

 than general degradation is carried on, the stratigraphic conditions of the 

 horizontal beds play a very important part in the method of degradation. 

 Here degradation by surface erosion is less and degradation by sapping 

 greater, and thus the walls of the canons retreat slowly in a series of steps 

 by this sapping process. Softer beds easily yield to atmospheric agencies, 

 while harder beds resist and stand in bold escarpments. 



Thus by faults and monoclinal flexures, by deep canons, and by lines 

 of cliffs the surface is cut into a great number of plateaus. 



In addition to the Plateaus proper, there are mountains due to upheaval 

 and degradation. The more important of these are the Zuni Range, to the 

 south, and the Uinta Range, far to the north. The Uinta Range is carved 

 from a broad upheaval having an east and west axis. On either flank of the 

 upheaval there is a line or zone of maximum displacement where the 

 upheaval is by flexure or by faulting. Between these zones there is a gentle 

 flexure either way to the axis. Thus the upheaval is in part by general 

 flexure from the axis as an anticlinal, and in part by faulting and monoclinal 

 flexure, as in the Kaibab structure. Again there are small areas which are 



