112 GEOLOGY OF THE DENVEB BASIN. 



THE REGION OF COAL CREEK TEAK. 



SCRFACE I'l'.A 1 I l:l.-. 



Structurally, the region to be described extends along the foothills from 

 a point a mile south of Coal Creek to one 2 miles north of it. Geologic 

 interest centers about Coal Creek Peak, a high, conical mass of granite 

 immediately north of the creek, that stands conspicuously forth from 

 tlir great body of Archean rocks to the west but with which it is still 

 connected by a long, narrow, granite ridge. The northern or northeastern 

 base of the peak is marked by a small stream which rises just within the 

 Triassic beds, and to the north of this, except tor local faulting, the normal 

 foothill topography prevails; the steeply inclined Red Beds form a lofty 

 ridge, separated by a sharply eroded gulch from the Archean hills; the 

 Dakota hogback confines a small valley in the Jura, and to the east come 

 in order the Benton, Niobrara, and Pierre formations. Opposite the peak 

 the topography completely changes; the granite stands well to the east of 

 the range proper, and the Trias, Jura, and Dakota almost wholly disappear. 

 The Dakota occurs only as a low comb well down on the face of the 

 mountain, in places disappearing altogether, the slope from peak to plain 

 being unbroken. Although the formation is present, it is not until nearly 

 2 miles south of Coal Creek that it reappears in hogbacks. The Red beds 

 of the Trias reappear at the northern bank of Coal Creek and south of the 

 stream again form a prominent ridge that finally constitutes the northern 

 spur of the Ralston Peaks, separated by a deep valley from the granite hills 

 to tlie west, and to the east directly overlooking the prairie. 



sti:c< TniAi. kfi.a thins of the formations. 



In the southern portion of the region under discussion the bed beds 

 successively disappear against the projecting mass of crystalline rocks, the 

 lowest first, at a distance of about a mile south of Coal Creek. The 

 measures are here overturned, the youngest the farthest, the Dakota at one 

 or two points showing a westerly dip as low as 25°. The fold is, however, 

 a comparatively sharp one, and it is probably hut a few hundred feet 

 below the surface that the normal easterly dip is attained. 



