FOOTHILLS STRUCTURE IN NORTHERN COLORADO 721 



LOCAL UNCONFORMITIES (SO-CALLED " ARCHES ") 



Certain peculiarities in the distribution of the geological 

 formations along the foothills, especially well developed near Golden 

 and Boulder, have been explained as the result of unconformities 

 due to local arching in the Cretaceous sea. 



The Golden area.- — The sketch map of the vicinity of Golden is, 

 with a single correction, the map by Eldridge given in his report 

 on "The Geology of the Denver Basin." 1 The areal distribution of 

 the formation has been well traced in this map and has proved 

 essentially correct, while the general features of the area in 

 question (Fig. 2) have been well described in the following 

 words : 2 



The topography shows a marked variation from that normal for the 

 foothills region in general. .... For mile after mile along the mountains the 

 normal topographical features may be traced with unswerving regularity, but 

 within the area to be described they undergo rapid change, and .... in the 

 vicinity of the town of Golden they are lost to recognition. For a distance of 

 over a mile north of the town, and an equal distance south of it, the Dakota 

 hogbacks have completely disappeared; the low Niobrara ridges cease to 

 exist at a point about a mile north of Bear Creek, not to appear again until the 

 region of Van Bibber Creek, 10 miles to the north, is reached; the Laramie 

 sandstones with their coal have gradually approached to within 500 feet of 

 the Archean at Clear Creek, the variation in their strike from that of the 

 Triassic and Dakota outcrops below being apparent to the most casual observer. 

 .... The lines of stratification are delineated clearly upon the surface and ■ 

 display a distinct tendency to group themselves, with respect to direction, 

 into two well-marked assemblages — the one embracing the formations of the 

 Colorado and all below, and maintaining for the greater part of their extent 

 the same parallelism to the general trend of the foothills which they have held 

 beyond the affected area; the other embracing the Montana and younger 

 formations, and though maintaining a parallelism of strike within themselves, 

 nevertheless abutting against the older formations, in fact approaching the 

 range proper in a broad, well-marked, and regular inward-sweeping curve, the 

 center of its arch lying a short distance north of Clear Creek. The features 

 just noticed again occur, in a minor degree and in a manner not at first liable to 

 attract attention, in the relations between the Dakota and underlying beds 

 nearer the middle of the area, where the beds of the younger formation lie 

 across the edges of those of the older. 



1 Op. tit., p. 83. 



2 Op. ciL, pp. 83-84. 



