HISTORY OF THE JURASSIC DEPOSITS 395 



red beds in most areas. Next came much deeper waters and extensive 

 clay deposition. At this time organic life was particularly abundant, 

 and thin layers of calcareous deposits were developed. Following this 

 came sandy deposition, culminating in the accumulation of beds now 

 represented by the Unkpapa sandstone, which, as above described, has 

 in places a thickness of 225 feet. Apparently the area of deposition 

 at this time was to the east and southeast, for there is no evidence of 

 degradation of the Unkpapa beds where the formation thins out to 

 the west. 



The Unkpapa sands were deposited in relatively quiet water, for the 

 material is uniformly fine-grained and rarely shows any current-bed- 

 ding. Next came the deeper waters in which were deposited a wide- 

 spread mantle of clay, now represented by the Beulah shales. Although 

 these shales are absent in the southeastern portion of the Hills, yet it is 

 probable that they were originally deposited there to a greater or less 

 thickness and then removed by erosion. This erosion resulted from 

 the uplift which constituted the next stage. How extensive the degra- 

 dation was is not known, but it has given rise to a general erosional 

 unconformity at the base of the Lakota sandstone, the next succeeding 

 deposit. 



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WATER LEVEL 



Figure 2. — Outline of History of Jurassic Deposition in Black, Hills Region. 



In figure 2 an attempt has been made to represent the principal stages 

 in the cycle of events above described. The diagram has little or no 

 quantitative status, but it shows the nature and general sequence of 

 events. 



As the post-Jurassic age of the Lakota formation is not yet definitely 

 ascertained, a few remarks concerning the character and histor}^ of this 

 series may be in order here. The materials are mainly coarse, cross- 

 bedded sandstones in thick masses, with intercalated shales, fine clays, 

 and local coal beds. They represent a time in which the waters were 



