IGO GEOLOCiY OF TUE BLACK IJILLS. 



based u])(>ii the Red Beds. The section is imperfect, but is added here to 



ilhistrate the thickness and character of the formation hi tliis part of the 



Kedwater Valley. 



Cretaceous. 



Feet. 



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



Jura. 



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



5. Sandstone, white and red . 15 



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



3. Clays or marls, greenish or gray ... 55 



Red Beds. 



2. (lypsum 6 



1. ("lay, deep red, with gj'psum to level of creek. . 1.30 



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

 to be the (;ase, the section gives a thickness of 400 feet to the formation, 

 and this, thoug-h 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 ^^alley widens 

 out greatly west of Spearfish 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 exceUent 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 Spearfish fronm the east, and turning northward follow its 

 east bank for some distance. Then crossing the Deepwater, 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 b}' 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 buttesin 



