160 GEOLOGY OF THE BLACK HILLS. 



based upon the Red Beds. The section is imperfect, but is added here to 



illustrate the thickness and character of the formation in this part of the 



Redwater Valley. 



Cretaceous. 



Feet. 



7. Sandstone, yellow, coarse and massive, fine grained ; conglomerate at base . . 70 



Jura. 



6. Talus, overlying (probably) beds of a soft nature 310 



5. Sandstone, white and red 15 



4. Sandstone, yellow, soft; containing calcareous bands 2 or 3 feet thick. 25 



3. Clays or marls, greenish or gray 55 



Bed Beds. 



2. Gypsum 6 



1. Clay, deep red, with gypsum to level of creek . . 130 



Assuming that the talus (6) is underlaid entirely by Jura, which seems 

 to be the case, the section gives a thickness of 400 feet to the formation, 

 and this, though greater than on the eastern side of the Hills, does not 

 surpass the thickness found a few miles north on the Redwater. As may 

 be seen by referring to the geological map the Redwater Valley widens 

 out greatly west of Spearnsh Creek, exhibiting the great development of 

 the Red Beds so frequently mentioned in the discussion of that series. 

 As the result of a study of the Jura in the foothills on the northern side of 

 Redwater Creek a number of excellent sections were obtained by Mr. Jen- 

 ney and Mr. Patrick which will relieve the monotony of the record of 

 imperfect exposures on the eastern side of the Hills and illustrate the fine 

 development of the formation on the northern. Here, too, the Jurassic is 

 quite fossiliferous, especially in two horizons, as will appear by the sections. 

 Referring again to the geological map it will be seen that the foothills 

 approach the Spearnsh from the east, and turning northward follow its 

 east bank for some distance. Then crossing the Deep water, they run 

 nearly westward for twenty miles, keeping parallel to the course of the 

 Redwater. The southern flanks of the foothills and frequently a consid- 

 erable portion of their summits are composed of Jurassic strata, and these 

 are overlaid by the Dakota sandstone which either caps the bluffs or appears 

 at some little distance on the northward slopes. Fragments of the Jurassic 

 rocks, with or without the cap of Cretaceous, are found in isolated buttes in 



