30 GEOLOGY OF THE BLACK HILLS. 



strata, now removed from the mountains by denudation, the ridges of which 

 Rawhide Butte is a type appear to be ridges of displacement following lines 

 of faulting — monoclinal ridges of displacement. Their trend, so far as 

 observed, is southeast and northwest, or nearly parallel with the axis of the 

 Black Hills, but the Laramie Mountains, running from Long's Peak, Colo- 

 rado, in a northerly direction to Laramie Peak, there make a great bend 

 toward the west, and in nearly an east and west direction extend to South 

 Pass, where they join the Wind River Mountains. 



Though the ridges undoubtedly belong to the period of elevation dur- 

 ing which occurred the main uplifting of the neighboring portion of the 

 Rocky Mountains and of the Black Hills, they are separate in themselves, 

 and, except as products of the same forces, form no connection between the 

 Black Hills and the Rocky Mountains proper. 



The valley of Old Woman Fork, along which our route was pursued, 

 is excavated from the beds of the middle and lower portions of the White 

 River Tertiary to within twelve or fifteen miles of its mouth, at which place 

 the underlying Cretaceous beds make their appearance. The Tertiary beds 

 consist of the same light gray or drab, argillaceous or calcareous sands, and 

 the banks of the valley are picturesquely sculptured into castellated bluffs 

 and buttes. 



Near Old Woman Butte the upper members of the Cretaceous appear, 

 well developed and containing" an abundance of characteristic and beauti- 

 fully preserved fossil forms. The basal portions of the Tertiary are also 

 well shown, w T ith the characteristic Titanotherium bed, full of vertebrate 

 remains. A somewhat imperfect section, showing the relation between these 

 formations near Old Woman Butte, is given below in descending order: 



Feet. 



7. White sandstone, coarse and incoherent, with quartz pebbles, and at base 

 6. A bed of coarse greenish sand, containing large quantities of the bones and 



teeth of Titanotheritm, Rhinoceros, &c 10-15 



5. Yellowish and grayish clay, capping two prominent buttes in the valley 5-8 



4. Brick red clay . 5-8 



3. Yellowish clay 10-15 



2. Coarse sand and conglomerate, quite ferruginous, with some limestone. 



Amount exposed above creek 20-30 



