50 DESCRIPTIVE GEOLOGY. 



mie Hills. A short distance east of here they pass under the Tertiary 

 plains. 



Between Horse Creek and Wahlbach Spring, on the Cheyenne Pass 

 road, the comjDlete series of foot-hill strata are represented, presenting a 

 rather uniform appearance. The Palaeozoic rocks form high abrupt bluffs, 

 somewhat resembling those to the northward, less bold and striking, but 

 with the same cliff-like face to the eastward, having a dip angle from 55^ to 

 65°. At the base of the cliffs, red clays and soils reveal the Triassic sand- 

 stone, although they fail to show the unbroken continuity of well-defined 

 strata that is exposed farther to the northward, while the Jurassic, which 

 here would appear to be quite thin, is indicated by a slight depression in 

 the formations, and a light clayey soil, capped by a light yellow sandstone. 



In the Colorado group, the subdivisions are not always well defined, 

 while the upper, or Fort Pierre beds, frequently lie concealed beneath the 

 Tertiary. Where the Fort Pierre beds are exposed, they have suffered 

 considerable erosion, and have been cut through by numerous streams and 

 ravines, which offer good sections of the dark carbonaceous shales and clays. 



Just north of Wahlbach Spring, the lower sandstones and limestones 

 occur inclined at 15° to 20°, possessing a much lower angle than the same 

 strata to the northward. The relations of the several horizons to each 

 other and their structural features are shown in the upper section at the 

 bottom of Map I, east half: in the section, however, the Niobrara Ter- 

 tiary beds jut up against the Dakota sandstone. A short distance to the 

 northeast, the overlying Tertiary strata have been eroded, and the yellow 

 chalky marls of the Colorado group are well exposed, carrying immense 

 quantities of the genus Ostrea. Interstratified in the marls are thin layers, 

 varying from 6 inches to 3 or 4 feet, of bluish clay-slates, with some 

 gypsum. 



Between the Cheyenne Pass road and the north branch of Crow Creek, 

 a broad elevated table-mountain extends out from the main Archaean range 

 to the eastward. On the east side it rises abruptly in nearly perpendic- 

 ular walls, for nearly 800 feet, wliile toward the main range it presents a 

 bluff nearly as precipitous, being separated from the granites by a deep 

 canon, which heads near the divide of Lodge Pole and Crow Creeks. This 



