Water Supply and Irrigation Papers of the United States Geological Survey . t are impressions of dicotyledonousleaves, regarded as Upper Cretaceous in age. BENTON GROUP. Graneros shale.—The Graneros shale, which is the lowest formationof the Benton group, extends entirely around the Black Hills uplift,with a course marked by lowlands and valleys. It consists of darkfissile shale 900 to 1,150 feet thick, the maximum amount being northof Belle Fourche. From Edgemont to Sturgis the average amount is900 feet. In some areas the formation contains, toward its base, a thin layerof hard sandstone, whi
Water Supply and Irrigation Papers of the United States Geological Survey . t are impressions of dicotyledonousleaves, regarded as Upper Cretaceous in age. BENTON GROUP. Graneros shale.—The Graneros shale, which is the lowest formationof the Benton group, extends entirely around the Black Hills uplift,with a course marked by lowlands and valleys. It consists of darkfissile shale 900 to 1,150 feet thick, the maximum amount being northof Belle Fourche. From Edgemont to Sturgis the average amount is900 feet. In some areas the formation contains, toward its base, a thin layerof hard sandstone, which rises in a ridge of considerable prominencenear Rapid, Hermosa, Newcastle, and Belle Fourche. It occurs locally,but apparently at the same horizon, from 150 to 300 feet abovethe Dakota sandstone. Its maximum thickness is about 30 Hermosa, where it is traceable from 4 to 5 miles, it is about 200feet above the base of the formation and attains a thickness of 15feet. At Rapid it is again conspicuous, but near Belle Fourche it isthin, soft, and in places GEOLOGY OF THE BLACK HILLS REGION. 21 Next above this sandstone horizon is a scries of hard gray shalesknown as the Mo wry shale member. It is from 100 to 250 feet thick,and owing to its hardness gives rise to ridges of moderate prominence,many of which bear scattered pine trees. The shales of this membercontain large numbers of fish scales and weather to a light silverycolor, both characteristic features. Greenhorn limestone.—One of the most prominent features in theplains immediately adjoining the Black Hills is a low but distinctescarpment, due to a hard limestone bed in the middle of the Bentongroup. It usually lies from 1 to 4 miles distant from the hogbackridge of the Dakota sandstone and presents its face toward the is thin but persistent, and is characterized by large numbers ofimpressions of Inoceramus labiatus, a fossil of scanty occurrence inthe adjoining formations. It is believed to rep
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