MOUNTAIN TOPOGRAPHY. 3 



loop, and has a trend NT. 20 W. as far north as Boulder Canyon, beyond 

 which it assumes a general direction X. 15 E. to the next projecting head- 

 land between tin- Thompson and Cache le Poudre rivers. 



In Paleozoic and Mesozoic times these indentations or bays formed a 

 much more prominent feature in Rocky Mountain topography than at 

 present, when they have become partially effaced 1>\ the sedimentation and 

 subsequent erosion which accompanied the successive subsidences and ele- 

 vations. Their orographic significance is hence much greater than would 

 appear al lir>t glance. They show thai the broader features of the present 

 mountain structure were blocked out in very early geological time, and 

 though these features have been modified locally l>\ successive orographic 

 movements and outflows of eruptive rocks it is upon them thai the present 

 drainage system is primarily dependent. 



The present eastward -flowing drainage of the Rocky Mountains follows 

 two greal river systems, the Arkansas and the Platte. These streams -ill 

 take their rise to the west of the main crest or Front Range <>\ the Rocky 

 Mountain system, and drain, respectively, one or more of the great interior 

 valleys, which are a characteristic feature "f its topography. 



The upper valley of the Arkansas is the must westerly ami also, in 

 geological structure, the youngest of these interior vallevs. The present 

 Arkansas River is not, however, necessarily "I youngt r formation than the 

 two main branches of the Platte, which drain the older depressions of 

 the South ami d!' the North Park, respectively. After following in a south- 

 erly direction the entire length of tin- Upper Arkansas Valley, which was 

 blocked out in late Mesozoic time, the present stream bends abruptly east- 

 ward, cutting deep canyons of comparatively recent formation through 

 preexisting ridges, the mosl remarkable of which is the Royal Gorge, 

 3,000 feet deep, and receives through Grape ('reek the drainage of Wet 

 Mountain Valley, which formerly went out in a southeasterly direction 

 through Huerfano Park. The location of this easterly course of the pres- 

 ent stream must have been originally determined at some time during the 

 Tertiary era. hut a verj large portion of the gorge cutting, especially 

 in the lower part of its course, mus1 have been accomplished since the 

 < rlacial epoch. 



