672 INDEX TO THE STRATIGEAPHY OF NORTH AMERICA. 



J-K 13-14. EASTERN COLORADO, KANSAS, AND NEBRASKA. 



Darton ^^^^ gives the following general account of the Cretaceous of the Great 

 Plains : 



Dakota-Lakota formations. — In 1893 Prof. Lester Ward discovered that the so-caUed Dakota 

 sandstone of the Black Hills contained not only a Dakota flora but in its lower beds extensive 

 flora of lower Cretaceous age. As the Dakota sandstone in its type region is characterized by a 

 distinct upper Cretaceous flora, it became necessary to restrict the term "Dakota" in the Black 

 Hills to the upper sandstone carrying the upper Cretaceous plants. In investigating the stratig- 

 raphy of the uplift it was found that the upper sandstone is separated from the lower sandstone, 

 which was designated the Lakota sandstones, by a persistent body of shale which has been desig- 

 nated the Fuson formation. In tracing these formations northward, it was found that the 

 principal plant-bearing horizon in the northern Black Hills was in the Fuson formation, which 

 has yielded a large and beautiful flora of lower Cretaceous plants, which Professor Ward has 

 described. The tripartite composition of the old "Dakota" group in the Black Plills is very 

 distinct throughout the uplift and apparently is a widespread feature in adjoining regions. 



Benton group. — The rocks of this group are the most widespread and constant in character- 

 istics of all the sedimentary deposits of the Central Plains region, their sahent feature being a 

 thick sv.ccession of shales overlying the Dakota sandstone. They present, however, persistent 

 subdivisions or horizons of variation. The thickness is variable, ranging from about 400 feet 

 in the southeast to 1,600 feet in the Black Hills. 



In nearly all the half million square miles under consideration the group comprises three 

 members — a basal, dark shale series known as the Graneros shale, a medial limestone known as 

 the Greenhorn limestone, and an upper shale series with sandy layers known as the CarUle forma- 

 tion. Toward its base the Graneros shale includes a horizon marked by local deposits of sand- 

 stone; the Greenhorn limestone always presents alternations of slabby limestone and shales; the 

 Carlile formation generally has a sandstone bed at or near its top, not far below which concre- 

 tions usually occur. The Greenhorn limestone is characterized by great colonies of Inoceramus 

 labiatus, a species rarely found at all in other horizons ; the upper portion of the Carlile contains 

 Prionotropis woolgari, which appear to be restricted to that horizon and to characterize it 

 throughout the region and even in the Bighorn Basin. Throughout east Wyoming and Black 

 Hills region the middle part of the Graneros shales includes, not far above the local sandstone 

 horizon a series of hard gray shales and fine-grained thin-bedded sandstones filled with fish 

 scales, which weather light gray and from their hardness often give rise to a ridge or cliff. These 

 have been termed the Mowrie beds and are conspicuous along both sides of the Bighorn uplift, 

 all around the Black Hills, and along the Laramie Front Range to the Colorado line. 



Nioirar a formation. — This deposit occupies a wide area in the central Great Plains region, 

 succeeding the Carlile without suggestion of unconformity and, except in the vicinity of the 

 Bighorn Mountains, consisting largely of carbonate of Ume. Its tliickness varies considerably 

 from apparently less than 100 feet in some portions of eastern South Dakota to 700 feet in central- 

 southeast Colorado. 



At the type locality on Missouri River at the mouth of the Niobrara the formation is repre- 

 sented by a chalk rock having a thickness of about 200 feet. In southern Nebraska and Kansas, 

 where it appears extensively, the amount is considerably greater, 350 feet being the estimate of 

 the Kansas geologists. The formation usually presents purer and harder carbonate of lime 

 deposits near its base, constituting the Fort Hays limestone in Kansas and the Timpas formation 

 in Colorado. The characteristic fossil of this horizon is the Inoceramus deformis, which is a 

 conspicuous feature in Colorado and for some distance north into Wyoming. 



