FOLDING 445 



rado. The Wasatch and Uinta ranges are separated by Kamas valley and 

 a group of irregular hills formed of extrusive andesite and breccia. In 

 the Wasatch range the fold is broken by the granite intrusion and asso- 

 ciated rocks forming the peaks and ridges about the headwaters of Little 

 and Big Cottonwood creeks. On the north side of this intrusive mass the 

 sedimentary rocks dip to the north and northwest and the anticline 

 pitches down beneath the Salt Lake valley. On the south side the steep 

 south or southeast dips change rapidly to an average dip of 5 to 10 de- 

 grees, and in American Fork canyon a secondary fold occurs with a 

 northwest-southeast trend. 



The axis of the Uinta fold trends a little north of east as far as Leidy 

 peak, and occupies a position north of the crest of the range (see plate 

 48). Prom this point the trend is east to the 0-wi-yu-kuts plateau. In 

 the Green River region the fold is broken by a thrust movement having a 

 general northwest-southeast strike and changing to east-west on both the 

 north and south slopes of the range. East of this region the fold is much 

 less prominent and is largely covered by later deposits. This portion of 

 the uplift has been described by C. A. White (5). 



OTHER SIMILAR FOLDS 



Until within recent years it has been supposed that the Uinta range 

 was the only fold in the Cordilleran region having an east-west trend. 

 Darton has described (9) a similar fold forming the Owl Creek range. 

 Eecent work in central Wyoming by the same geologist has shown the 

 occurrence of several folds having the same trend. 



While the major folding of the eastern and central Cordilleran region 

 varies in trend from north-south to northwest-southeast, there was a con- 

 siderable crumpling of the crust in the opposite direction. So far as 

 observations have been made, both movements seem to have occurred 

 during approximately the same geologic period. 



AGE OF THE FOLDING 



It appears to be generally considered that the orographic movement 

 which in great measure determined the position and direction of the 

 Eocky Mountain and Central Cordilleran ranges developed after the depo- 

 sition of the last marine Cretaceous strata and prior to the deposition of 

 the fresh-water Eocene sediments. Observations during recent years in- 

 dicate that this interval represents a long period of geologic time. There 

 was profound faulting and folding and upthrusting of the crust which 

 mark this as one of the great periods of mountain building of the earth's 



