THE BUILDING OF THE COLORADO ROCKIES 



149 



Rockies proper are several scattered mountain groups, such as the 

 San Juan Mountains, Elk Mountains, West Elk Mountains, and 

 Flattop Mountains, which mark spots of excessive Tertiary vul- 

 canism. Structurally they are quite distinct from the folded 

 Rockies proper, and occur as isolated groups in the midst of com- 

 paratively undisturbed, fiat-lying sedimentary rocks. There are 



Fig. i.^ — Geologic map of Colorado (after R. D. George). The darkly shaded 

 pre-Cambrian areas delineate rather closely the pattern of the Rocky Mountains 

 within Colorado. The ranges converge southward in en echelon arrangement roughly 

 suggesting the letter Y . 



also some local domelike upwarps in the western part of the 

 state. These local upwarps and the isolated groups genetically 

 related to Tertiary vulcanism should be sharply differentiated from 

 the Rocky Mountains proper — the folded range. The history 

 of the impressive San Juan group has been worked out in detail 

 by Cross,^ Atwood,^ and others. 



I Whitman Cross, U.S. Geol. Survey, Geol. Atlas, Needle Mountains, Rico, Silver- 

 ton, Telluride, and other folios. 



^W. W. Atwood, "Eocene Glacial Deposits in Southwestern Colorado," U.S. 

 Geol. Survey, Prof. Paper gj-B (1915), pp. 22-24. 



