570 INDEX TO THE STRATIGRAPHY OF NORTH AMERICA. 



limestone, which, is supposed to represent the top of the Chugwater red beds, there are 20 feet 

 or more of white to red sandstones, then a few feet of buff sandstones and shales with an 8-foot 

 bed of gypsum, and finally an upper series of dark shales with limestone layers and concretions 

 filled with characteristic fossils, including many Belemnites densus. 



Along the North Platte River, in its big bend south of Douglas and to the west on Wagon- 

 hound Creek, the succession is as follows, from the bottom up : 30 feet or more of massive gray 

 sandstone; 30 feet of pale-greenish sandy shales; 5 feet of soft greenish massive sandstone; 40 

 feet of bright-reddish sandy beds; 15 feet of massive buff sandstone; and at the top about 200 

 feet of green shales with a few thin beds of sandstone and limestone containing many upper 

 Jurassic fossils. 



The formation appears again on Chugwater and Horse creeks, consisting of an upper 

 series of 30 feet of soft slabby sandstones, gray at the base and buff toward the top, with ripple- 

 marked layers and much intercalated shale, and a lower series 50 feet thick, mostly of green and 

 gray shale, with layers of soft thin-bedded greenish-gray sandstones lying on the Chugwater 

 formation. * * * 



This formation appears on the banks of North Platte River northwest of Guernsey and for 

 some distance northward. The lower 145 feet are sandstones with a few thin beds of shales, 

 over which there are 60 feet of interbedded slabby sandstones and clays, all containing charac- 

 teristic marine Jurassic fossils. * * * 



The Sundance formation extends only a few miles into Colorado from the northward, finally 

 ending by thinning out. Dr. Hayden found fossils, ' ' Ostrea and fragments of Pentacrinus aster- 

 iscus," on Box Elder Creek, in yellow sandstones and clays, with scattered layers or nodules of 

 limestone. He suggested that the limestone sometimes found at the base of the Morrison 

 formation may be a representative of the marine Jurassic, a suggestion based on its similarity 

 in character and relations to a limestone on the Laramie plains, which contains Apiocrinites. 

 As similar limestones exist in typical Morrison beds to the northward, where the marine Jurassic 

 is represented, I feel certain that all those in the Front Range are of Morrison age. * * * 



The Jurassic appears to exist only in the northwestern portion of the region to which this 

 report relates, apparently owing to nondeposition in other portions of the region. In the Big- 

 horn Mountains and Black Hills it is represented by from 300 to 400 feet of deposits, but these 

 thin gradually to the southward in the Laramie Range and disappear in the northern portion 

 of Colorado. The thinning appears to be general at the outset and the upper beds probably 

 disappear first, but this point has not been definitely determined, and it may be that the 

 upper beds merge into sandy beds and these thin out gradually together with the underlying 

 sandstones. 



The formation is evidently of marine origin, as indicated by its ntunerous molluscan remains, 

 and its age is regarded as late Jurassic. It has not been divided into subordinate members, 

 but in its regular succession it presents a succession of beds and faunas which are constant over 

 a wide area, especially the sandstone near the base and the green shales above containiag numer- 

 ous Belemnites densus. It is probable that the Sundance formation does not extend far east 

 of the Black Hills, nor to the southeastward of the locality at which it disappears in surface 

 outcrops in northern Colorado, but there is no direct evidence on this question. 



The Unkpapa sandstone, which succeeds the Sundance formation along the eastern side 

 of the Black Hills, is a relatively local feature of unknown age. It appears to represent a local 

 shore deposit in late Jurassic times', prior to the deposition of the Morrison beds. The horizon 

 may possibly be represented in other regions by the almost general occm-rence of a yellowish 

 sandy bed at the top of the Sundance formation. If not, it is probable that in the area in which 

 it is absent there is a small unconformity or hiatus at this horizon. There is a very abrupt 

 change from the Sundance to Morrison sediments, but no direct evidence of unconformity has 

 ever been found. 



The latest notes on the Jurassic fauna of the Black Hills are given by Whitfield 

 and Hovey."^^ Summary statements concerning the correlation of the faunas are 

 given by Stanton.''^^ 



