164 GEOLOGY OF THE BLACK HILLS. 



occupy a broader area. Southwest from Sun Dance Hills, and between 

 them and the foothills, many fragmentary portions of the Jura are seen in 

 the Red Valley. 



Following the Jurassic outcrop westward and northwestward to the 

 valley of the Belle Fourche one finds there the greatest vertical develop- 

 ment of the formation in the Black Hills country. Around the main por- 

 tion of the Hills its thickness, so far as ascertained, is 200 to 300 feet ; 

 along the Redwater Valley it is a little more than 400 feet, and on the 

 Belle Fourche between 500 and 600 feet. 



If the reader will refer to Figure 24 he will readily understand the 

 structure of the Belle Fourche Valley in the vicinity of Bear Lodge. In 

 the lowest part is the immediate valley or bottom of the river, carved from 

 the Red Beds. On each side of this rises a bluff, showing Red Beds at 

 base and Jura above. Above the bluff is a terrace on each side, floored 

 by the local hard sandstone of the Jura. Back of this (on each side, though 

 represented on one only) is a second bluff or cliff exhibiting Jurassic strata 

 at base and Cretaceous at top. The strata are arched in a direction at right 

 angles to the line of section, so that if one follows the river far enough in 

 either direction he will find the Jurassic terrace to gradually descend until 

 it finally clips entirely beneath the water level. Where this takes place the 

 cliffs capped by the Cretaceous approach the stream so closely that they 

 are separated only by the bottom land. 



In the illustration the vertical scale is greater than the horizontal. In 

 reality the face of the lower cliff is 300 feet high and the face of the 

 upper about 500 ; the river bottom is one or two miles broad and the ter- 

 race three or four miles. It must not be understood that the terrace is 

 regular and continuous. It is indented by numerous lateral valleys and on 

 the divides between them salients of the upper cliff encroach more or less 

 on its area. 



On the east side of the Belle Fourche, about four miles southeast from 

 Bear Lodge, the following section was made, beginning with the summit 

 of the cliffs and their capping of Cretaceous sandstone, and showing the 

 entire thickness of the Jura: 



