388 N. H. DARTON STRATIGRAPHY OF THE BLACK HILLS, ETC. 



lower sandstone appears in a prominent cliff. Fossils occur also in the 

 sandstone and all are of later Jurassic age. At the base of the forma- 

 tion there is often a massive red or buff sandstone occurring in extended 

 lenses, and frequently attaining a thickness of 25 feet, lying unconform- 

 ably on Spearfish red shales, as shown in plate 25, figure 2. 



UNKPAPA SANDSTONE 



This formation extends all along the eastern side of the uplift, but has 

 its greatest development at the extreme southeast. The rock is a sand- 

 stone, characterized by its fine grain and very massive bedding, and 

 varies from white to purple and buff in color. It reaches its greatest 

 thickness of 225 feet south of Hot Springs, but is thin northward, except 

 in the vicinity of Rapid City, where it thickens to 150 feet. It appears 

 to be conformable to the underlying Sundance formation, but begins 

 abruptly without transition beds. 



The formation has not yielded fossils. It is a product of beach and 

 shallow water deposition, either at the close of the Jurassic or the be- 

 ginning of early Cretaceous time. It is provisionally classed in the 

 former. 



On the western and northwestern side of the Black hills a thin bed of 

 buff sandstone occurs at the top of the Sundance formation, which may 

 represent the western extension of this formation. In this case it may 

 possibly constitute a bed of passage from the Sundance to the Morrison 

 formation. 



MORRISON SHALES 



This formation has been known as the "Atlantosaurus beds " and 

 Beulah shale. It is extensively developed in the Black hills, excepting 

 to the southeast, where it is absent, and its place apparently is repre- 

 sented by an unconformity by erosion on the surface of the Unkpapa 

 sandstone. 



The Morrison deposits are mainly massive shale or "joint clay," some- 

 what more fissile and darker to the east than to the north and west. 

 The predominant color is a light greenish gray merging into chocolate 

 and maroon tints. Thin beds of fine grained, white or light gray sand- 

 stone are included, and some thin local layers of impure limestone. 

 A few fresh-water shells have been observed, and an almost general 

 occurrence of saurian bones, believed to be of earliest Cretaceous age, 

 although some paleontologists regard them as late Jurassic. Where the 

 formation lies on the Unkpapa sandstone and Sundance shales, there is 

 apparent conformity and rapid change from one material to the other, 

 but if the Unkpapa sandstone is not represented in the area in which 



