80 GEOLOGI oh' Till-; DENVEB BASIN. 



In the following description the foothill region includes nol only the 

 ,-,.,: foothills proper, 1 >u i as much of the adjoining prairie — usually 



a lu'li aboul 2 miles wid< — as lias lieen involved iii the 

 structural developmenl of the former. The prairie region 

 /)•• includes all else. Areas of specialized structure in the foot- 



hills lie aboul Golden, Boulder, the Smith Boulder peaks, and 

 Coal Creek. Within the prairie region there is Put a single 

 specialized area, the Boulder Valley. 



THE FOOTHILLS. 

 GENERAL STKI CTl'lIK. 

 Normni urrcorancc. — The general strnetiiiv of the foothills is 



shown in the several transverse sections of the field (l'l. IV) 

 h Somewhat greater detail is afforded in the section at Deer 

 Creek (fig. 3), excepl that here the fold in the Archean and 

 Trias, near the western limit of the latter, is a local feature. 

 The normal appearance of the foothills is that of a moun- 

 tain mass of Archean rocks, fringed at an average distance of 

 one-half or three-quarters of a mile by a sharp serrated ridge 

 of Dakota sandstone, the valley between the two being occu- 

 pied l>\ the formations of the Trias and Jura. Above the 



— I 



Dakota come, in their geological succession, the Benton, the 

 Niobrara — this generally constituting a second, smaller reef 

 outside the Dakota — the Pierre, the Fox Hills, and the Lara- 

 mie, tlie basal sandstones of the Laramie again forming either 

 a low roll in the ground or an actual comb >>l rock slightly 

 projecting above the surface oi the surrounding prairie. To 



the east of the Laramie, at a distance of between 600 and 

 1,200 feet from its basal sandstone, appears in the southern 

 portion of the area vet another comb, formed by the con- 

 glomerates at the base of the Arapahoe series. Finally, this 



is followed at aboul an equal distance by either an outcrop of 



the lower members of the Denver formation or a peculiar 



ribbing of the prairie due to their presence beneath the surface. 



