394 N. H. DARTON — JURASSIC FORMATIONS OF THE BLACK HILLS 



lying between the Unkpapa sandstone and tlie Lakota formation. They 

 thicken rapidly in their extension northward and pass around the 

 northern and western side of the Hills as a prominent member of the 

 series. 



Beyond the edge of the Unkpapa sandstone they lie conformably 

 on the Sundance formation, and owing to the similarity of materials 

 might not be readily separated if their true relations had not been de- 

 termined on the east side of the Hills. They extend along the western 

 side of the uplift and finally thin out north of Edgemont. The ma- 

 terial of the formation is mainly a dark-colored shale, much more fissile, 

 and darker to the east than to the north and west. From Sundance to 

 Cambria and Minnekahta they are a light greenish-gray, somewhat 

 massive mixture of gray clay and sand. They are moderately consoli- 

 dated and crumble more or less on weathering. In this area some of 

 the beds exhibit purple tints. A few thin sandstone beds are included 

 throughout. 



History of the Jurassic Deposits 



The geologic history of the Jurassic deposits in the Black Hills region 

 can be outlined in a general way. They represent conditions of deposi- 

 tion intermediate between those under which the Red beds were laid 

 down and those which gave rise to the Lakota and Dakota sandstones- 

 The Black Hills have been a nucleus of uplift and subsidence for a long 

 period, but the sequence of events appears to have been about the same 

 as in the Rocky Mountain region in general. At the end of the period 

 in which the Red beds were deposited, there was an uplift of the entire 

 Black Hills area, with slight erosion, mainly' by planation of the red 

 deposits. Then followed submergence, with sandy shores on which ac- 

 cumulated the materials of the massive red sandstones that are now 

 exposed as the basal Jurassic beds in a portion of the region. As this 

 submergence continued argillaceous deposition became almost general, 

 and the materials of the lower shale series were laid down. Early in 

 this stage there were deposited the thinly bedded sandstones containing 

 the fish described by Doctor Eastman. 



The next stage was an arrest in the submergence and more vigorous 

 conditions of erosion and deposition, giving rise to the beds now repre- 

 sented by the widely extended series of buff, slabb}^ sandstones usuall}'' 

 prominently ripple-marked. In the next stage the conditions were 

 much more diverse in different portions of the region, affording deposits 

 now represented by alternating shales and sandstones with included 



