DAKOTA SANDSTONE 391 



hogback range; but it is a conspicuous formation, because the foothills 

 which usually mark its outcrop rise steeply out of the lowlands of shale. 

 It generalh 7 consists of brown sandstone, hard and massive below, as 

 shown in figure 2, plate 26, but thinner bedded above. It presents little 

 variation in thickness and character. The rock is mostly coarse grained 

 more or less cross-bedded, and usually contains more oxide of iron than 

 the Lakota sandstone. It contains fossil plants which are impressions 

 of characteristic dicotyledonous leaves of the Dakota or Upper Creta- 

 ceous flora. 



GRAN EROS FORMATION 



This formation, which is the lowest member of the Benton group, out- 

 crops in the broad band of lowlands encircling the hogback range of the 

 Black hills. It has an average thickness of about 900 feet, consisting 

 mostly of fissile black shales, though in some districts there is included 

 a bed of sandstone from 1 to 30 feet thick, at a horizon from 200 to 300 

 feet above the base of the formation. This sandstone is noticeable in 

 the vicinity of Newcastle where it contains some petroleum, near Rapid 

 and Hermosa, on the east side of the hills, and at a few points in the 

 northern foothills. At a horizon a short distance above this sandstone 

 horizon there occurs a very distinctive series of hard, sandy shales, from 

 50 to 300 feet thick, which extends entirely around the uplift. These 

 shales are characterized by weathering to a dull silver gray color, by 

 containing large numbers of fish scales and by usually giving rise to a 

 low but distinctive ridge bearing a stunted growth of pines. They at- 

 tain great prominence near the Belle Fourche, at the northern end 

 of the uplift, where, according to observations by Professor O'Harra, 

 their thickness is 300 feet and their top lies 200 feet below the summit of 

 the Graneros formation. At Newcastle, in the central-western side of 

 the uplift, this series is thin and lies about 700 feet below the top or just 

 above the sandstone lens. The overlying deposits are dark gray shales 

 throughout the region. 



GREENHORN LIMESTONE 



This is an impure limestone lying near the middle of the Benton 

 group, which gives rise to a low but distinct escarpment extending 

 around the Black hills. The bed averages about 50 feet thick and is 

 everywhere characterized by large numbers of impressions of Inoceramus 

 labiatus, a fossil of infrequent occurrence in the adjoining formations. 

 It appears to gain hardness on weathering, breaking out into hard, thin, 

 pale buff slabs, generally covered with impressions of the distinctive 

 fossil. At its base it is usually distinctly separated from the dark shales 

 of the Graneros formation, but it grades upward into the overlying for- 



