392 N. H. DARTON — STRATIGRAPHY OF THE BLACK HILLS, ETC. 



mation through 6 or 8 feet of passage beds. In some portions of the 

 northern hills it is less distinct and less fossiliferous than to the south- 

 ward. About Edgemont and east of Hot Springs it is a prominent 

 feature. A typical exposure showing the alternation of thin-bedded lime- 

 stones and the abrupt change between Graneros and Greenhorn deposits 

 is shown in figure 2, plate 27. 



CARLILE FORMATION 



This upper member of the Benton group consists mainly of shales, 

 with occasional thin beds of sandstone, and at the top contains numerous 

 oval concretions. The thickness varies from 500 to slightly over 700 feet, 

 the larger amount being along the west side of the uplift. The following 

 section of Carlile formation, 3 miles west of Newcastle, is typical : 



Feet 



Niobrara chalk 



Dark shales, with light colored concretions. . 130 



Dark shales 200 



Calcareous concretions 3 



Sandy shales, with thin sandstones 170 



Brown sandstone . 4 



Greenhorn limestone 



Typical Benton fossils occur at intervals in the Carlile formation, 

 mainly in thin beds of limestone or in the concretions of the upper 

 series. The concretions of the upper series of the formation are espe- 

 cially characterized by the occurrence of the Prionotropis woolgari. 



NIOBRARA FORMATION 



The calcareous deposits of this formation completely encircle the Black 

 hills, but in a few districts to the northwestward they are not as dis- 

 tinctive as in other portions of the uplift. The typical material is a 

 light gray, soft shaly limestone or impure chalk, containing greater or 

 less admixture of clay and fine sand. Its weathered outcrops have a 

 bright yellow color, which usually renders them conspicuous, although, 

 owing to the softness of the materials, they rarely give rise to notable 

 ridges. The thickness averages about 200 feet, except to the northwest, 

 which is about 100 feet. Thin layers of hard limestone are often in- 

 cluded, consisting of an aggregation of shells of Ostrea congesta, a fossil 

 distinctive of the formation when it occurs in colonies. 



PIERRE SHALE 



The Great plains area adjoining the Black hills is occupied mainly by 

 the Pierre shale, a formation consisting of a thick uniform mass of dark 



