CEETACEOUS WEAR THE BLACK HILLS. 33 



or hog-backs is formed of the sandstone of the lower Cretaceous, directly 

 overlying the softer Jurassic beds. 



From the Cheyenne Kiver northwestward to Camp Jenney on the east 

 fork of the Beaver,' several sections were obtained across the hog-backs. 

 Thus, on the Cheyenne, two or three miles below the place Avhere the sec- 

 tion last given was obtained, the following section of the upper Cretaceous 

 was observed : 



Feet. 



5. Bed ferruginous sandstone, sometimes conglomeratic, capping the bluff and 



dipping 5° westward . 10 



4. Sandy shales 40 



3. Bed sandstone, full of beautifully preserved shells 5 



2. Soft yellowish sandstone (exposed) 30 



I. Concealed to base of cliff 100-150 



These beds are shown by their fossils to belong to the Fox Hills group, 

 or No. 5 Cretaceous. Eastward from these outcrops the surface of the coun- 

 try for a considerable distance is undulating, exhibiting but few sharp expos- 

 ures — such a contour as would naturallv arise from the soft character of 

 the underlying dark clays of the Fort Pierre group, No. 4 of the Creta- 

 ceous. In the valley of Beaver Creek, near the junction of its east and west 

 forks, are exposed excellent sections, capped by the highly fossiliferous, 

 calcareous strata of the Niobrara group — No. 3 of the Cretaceous series — 

 full of fragments of Ostrea congesta and Inoceramus. 



Feet. 



II. Shaly limestone, filled with fragments of Ostrea and Inoceramus, dipping 



20° west and capping the bluff 30 



10. Gray shale or marl, with small limestone concretions, calcite crystals, and 

 gypsum in thin layers or seams; becomes reddish and yellow, changing to 



gray or drab again at base 120 



9. Shaly limestone, with Gryphcea and fish teeth 2 



5. Bluish or drab clay shale or marl, becoming nearly black below, with much 



alkaline matter and several bands of carbonate of iron, 10 or 12 niches thick 



(estimated) 150-200 



7. Shaly sandstone 5 



6. Sandy and argillaceous shales, with calcite 30 



5. Shaly sandstone 20 



4. Drab clay shales with cone-in-cone concretions 20 



3. Gray or drab, clay or marl - - 20 



2. Shaly limestone, with Inoceramus fragments 8 



1. Black alkaline shale, with some shaly sandstone, some iron-ore, limestone 



nodules, calcite, and gypsum, to Beaver Creek 100 



3bh 



