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



have such imperfect knowledge of the significance of many of the rocks 

 and their relations. 



Black Hills Region 



list of the forma tions 



There are presented at the surface in the Black Hills uplift a series of 

 formations from middle Cambrian to latest Cretaceous in age, all of the 

 periods being represented in whole or in part, except Silurian, Devonian, 

 and Eocene prior to Oligocene. In the table on page 383 are given the 

 formations of the Black Hills uplift and some of their principal features. 



DEAD WOOD FORMATION 



The outcrop of this formation encircles the Black hills, appearing 

 usually in the base of the great in-facing limestone escarpment. It lies 

 on a relatively smooth surface of Algonkian rocks, although there are 

 many local irregularities of shorelines and beach phenomena. The 

 rocks are mostly sandstones and sandy shales, with frequent occur- 

 rences of basal conglomerate. In the southern hills the thickness varies 

 from 4 feet to 50 feet in greater part, and the principal material is a 

 coarse, dark brown, massive sandstone. To the northward the forma- 

 tion gradually thickens, apparently by the addition of higher beds com- 

 prising dark gray shales, mostly sandy, and beds of sandstone. In the 

 region about Deadwood, where the formation attains its greatest thick- 

 ness, of over 400 feet, it comprises about 30 feet of basal conglomerate 

 overlain by 30 feet of coarse, dark brown sandstone ; 200 to 400 feet of 

 gray shales, with layers of flaggy limestone, limestone conglomerate, 

 and sandstone ; a conspicuous member of hard, massive sandstone 5 

 to 12 feet thick, and at the top 20 to 45 feet of green shales. The 

 limestone conglomerate is a verj- characteristic rock, consisting of flat 

 pebbles and flakes of limestone more or less thickly sprinkled with 

 glauconite grains, and is of the intraformational type. A typical ex- 

 posure of Deadwood sandstone, at Deadwood, South Dakota, is shown 

 in plate 24. 



Throughout its course the formation contains fossils of which the fol- 

 lowing middle Cambrian forms have been reported by C. D. Walcott : 

 Obolus, Hyolithes, Dicellomus, Asaphiscus, Olenoides, Ptychoparia, and 

 Acrotreta. 



WHITEWOOD LIMESTONE 



This formation is a conspicuous member in the northern Black hills, 

 particularly about Deadwood, where its thickness is 80 feet. Its name 

 was given by T. A. Jaggar, Jr., from typical exposures in Whitewood 



