182 GEOLOGY OF THE BLACK HILLS. 



At the pass of Red Canon Creek the rocks which immediately overlie 

 the Dakota sandstone are : 



Feet. 



1. Black clay with much seleiiite 60 



2. Impure sandstone 2 



3. Black and gray clays 50 



At the pass of the east fork of Beaver Creek, the outer slope of the 

 Dakota, dipping westward at an angle of 35°, is overlaid by the black alka- 

 line clays of the Fort Benton 150 or 200 feet thick. From these have been 

 excavated the small valley of the "Bitter Fork," the waters of which are 

 highly charged with dissolved salts. A small ridge immediately west of the 

 valley exposes the following section, resting directly upon the Dakota 

 sandstone : 



Feet. 



7. Gray clay shales . 20 



6. Sandstone, calcareous, lamellar, brown, with a very fossiliferous layer, con- 

 taining imperfect specimens of Ammonites, teeth of Lamna, Otodus, etc 5 



5. Dark gray clay ■ 15 



4. Sandstone, lamellar and brown 4 



3. Dark gray clay shales 20 



2. Sandstone, lamellar and brown 10 



1. Dark gray or black clay shales with selenite and alkaline salts 75 



Westward from this point to the west fork of the Beaver are frag- 

 mentary exposures of the gray or yellowish clay of the Niobrara without 

 discovered fossils, and then, in barren slopes along the Beaver, the colored 

 clays of the Fort Pierre group, containing many calcareous bowlders or 

 concretions filled with Inoceramus, Ammonites placenta, Baculites, ScapMtes, 

 Helicoceras, Heteroceras, Ancyloceras, and Ptyclioceras. 



The Cretaceous exposures on the lower Beaver, on the Cheyenne 

 above its mouth, and on Old Woman Fork have been described on pre- 

 ceding pages. On the northwest side of the Hills the Fort Benton and 

 upper groups were not examined. The most extended and continuous 

 examination of the formation was made during the exploration of the Belle 

 Fourche from Bear Lodge to its junction with the south fork of the Chey- 

 enne, and the ascent of the latter stream to the mouth of French Creek. 

 For this whole distance the river runs tortuously in a valley from one to 

 two miles in width, cut in the soft rocks of the different groups of the Cre- 



