FOOTHILLS STRUCTURE IN NORTHERN COLORADO 717 



case (at Golden) even within 4,500 feet from the contact; while 

 in the northern part of the area dips of 15 ° east are found as much 

 as 10 miles east of the Archean sedimentary contact. 



These contrasts in dips indicate that the foothills fold differs 

 in shape and intensity over this area. The generally low dips 

 near Fort Collins and Loveland indicate an ideal, fairly gentle, 

 monoclinal fold, while the steep and overturned dips to the south 

 indicate an S-shaped fold of pronounced type. The fold proper is 

 well described by Fenneman 1 as follows : 



The master structural feature of this region is the great upturn of the 

 strata against the mountain range. .... The first Archean belt west of the 

 foothills is a dissected plateau .... from 6,500 to 7,000 feet above sea-level at 

 its eastern edge, where it ends abruptly and is flanked by the Fountain sand- 

 stone The height .... above the plains is nearly 1,000 feet 



Five miles east of the base of the foothills the Archean surface is at least 9,500 

 feet below the surface, or 4,200 feet below the level of the sea. The real face 

 of the granite plateau is therefore about 2 miles high, and this enormous rise 

 is accomplished in 6 miles. 



The opinion is also expressed that the greater part of the 

 monoclinal flexure is the result of subsidence during the deposition 

 of the various sedimentary formations, a conclusion passed on the 

 assumption that the ancient shore line was located along the line of 

 the present Front Range. 



This generalization was, however, based on a small area of the 

 foothills in which the characteristic structure is not well developed. 

 Further, Lee 2 has proved since that all the Cretaceous formations 

 up to and including the Laramie formerly were continuous over the 

 present site of the Front Range. Consequently their present atti- 

 tude must be due to orogenic forces, and the statement that "the 

 process of mountain making gave to the granite plateau west of the 

 foothills a comparatively small relative uplift above the plains" is 

 erroneous. 



A careful study of the dips and strikes of the formation south of 

 the Boulder area and the reconstruction of a fold based on these 

 dips as well as the occurrence of the lower formations as "inliers" 



1 Op. cit., pp. 41, 42. 



2 U. S. Geol. Survey, P.P. 95-C. 



