184 GEOLOGY OF THE BLACK HILLS. 



sidurable quantities of soluble salts. Its full thickness was not ascertained, 

 but a cliff cut in it was found to be nearly 200 feet in height. 



The surface of the country north of the river is gently undulating, 

 and but little scored by branching streams or ravines, so that along almost 

 the immediate bluff of the valley wagons can be driven for a long distance 

 without arrest by irregularities of the ground. It is destitute of water, and 

 for long stretches almost destitute of vegetation. The same clay and the 

 same surface features characterize the country bordering the river for along 

 distance west of the mouth of the Redwater. 



Some little distance below Bear Butte Creek the Fort Pierre group is 

 well distinguished as a gray clay, with calcareous nodules full of charac- 

 teristic fossils. It was not fully connected with bed 4 of the last section, 

 but its identity is highly probable. Among the fossils are Ammonites pla- 

 centa, Baculites, and Inoceramus Sagensis. 



Near the forks of the Cheyenne the following beds were found — 



Feet. 



3. Yellow sand or arenaceous clay 20 



2. Clay shales, light gray, darker in streaks, with impure limestones at base and 



a few limestone coucretions with fossils (No. 4) 110 



1. Dark gray clay shales, with calcareous nodules full of Baculites, Inoceramus, 



Ammonites, etc. (No. 4) thickness to river. . 75 



Beds 1 and 2 of the above section belong evidently to the Fort Pierre 

 group, while bed 3 may possibly represent the Fox Hill. 



Between the Cheyenne and the Hills a large area of country is under- 

 laid by the Fort Pierre group, frequently exhibiting its characteristic 

 barrenness of surface. 



From the forks of the Cheyenne up the south branch to and beyond 

 the mouth of French Creek the river is cut in the dark gray clays of the Fort 

 Pierre group, and from the abundant limestone nodules we obtained many 

 fossils. When freshly exposed these concretions are broken with the greatest 

 difficulty, and in the operation the fossils are generally injured or destroyed. 

 When, however, they have been exposed for some time and are somewhat 

 decomposed they are readily broken, and the fossils with their beautiful 

 nacre and colors are obtained with ease. In the beauty and abundance of 



