390 



STRUCTURAL GEOLOGY OF NORTH AMERICA 





Fig. 25.1. Index map for cross sections of Colorado and New Mexico. Refer to Raisz' Landforms 

 Map of the United States for location of ranges, mountains, and valleys. Stippled area is zone of 

 marked laramide disturbance. 



of later Cretaceous shales and sandstones, was considerably deformed in 

 Laramide times. The evaporites contributed to great distortion of the 

 beds in die Eagle area (Fig. 25.4), and the White River uplift or arch as 

 a continuation of the Uinta uplift became the largest structure in the 

 ancestral basin. 



Central Parks 



Although Middle Park is a well-defined drainage basin, it is more 

 complex topographically and structurally dian the other parks. North 

 and South parks have little, or only moderate, internal topographic relief; 

 structurally they are broad, open synclines, marginally faulted, particu- 

 larly along the eastern sides. Middle Park is also generally synclinal, but 

 the structural and topographic continuity is disturbed by many projecting 

 mountain spurs characterized by overthrust faulting. Northward trending 

 spurs of the Front Range, comprising the Williams River Mountains and 

 the Vasquez Mountains, extend into the southern part of the park as far 

 as the Colorado River. The northern part is largely occupied by spurs 

 projecting southward from the Never Summer Mountains and from 

 the unnamed ridge followed by the continental divide between Middle 

 and North parks. 



Front Range 



The Front Range is mostly an expanse of Precambrian rock with up- 

 turned or overthrust sedimentary rocks on east and west flanks. The Pre- 

 cambrian rocks have been reviewed in Chapter 4. According to Lovering 

 and Goddard (1950): 



The western side of the Front Range is marked by a series of great over- 

 thrust faults that formed at this time from the southern end of the South Park 

 as far north as the Wyoming line. The displacement on the Williams Range 

 thrust fault north of Breckenridge is more than 4/2 miles, and the movement 

 on the Never Summer thrust north of Branby is more than 6M miles. The 

 eastern side of the Front Range was subjected to much less severe deformation 

 but was the locus of many echelon northwesterly folds and persistent steep 

 northwesterly faults. Its structure is dominantly that of a steep monoclinal 

 fold, though locally, as at Colorado Springs and Boulder, some thrusting has 

 taken place. 



