136 GEOLOGY OF THE BLACK HILLS. 



summit of the foothills that encircle like a rampart the fastnesses of the 

 Black Hills. Looking back from this elevation he commands a broad 

 stretch of the Plains, a grassy sea, quietly undulating in the distance, but 

 broken near his feet into rocky billows. Facing about, he finds that he is 

 standing upon the verge of an escarpment 200 to 500 feet high, and beneath 

 him is the undulating, treeless Red Valley stretching away to the right and 

 left until by its curvature it is carried out of sight. Beyond it and two 

 miles, or four miles, or six miles away, the main mass of the Hills rises 

 darkly against the sky, timbered and rocky. Just at the base of the Hills, 

 and parting them from the valley, runs the bare outcrop of the purple lime- 

 stone, not an even band, but a serrated fringe, divided by every water-course 

 that descends to the valley and pointing a sharp crag upward in every 

 interval. 



This structure of the Red Valley is indicated in the several cross-cuts 

 in subsequent portions of this section, Figures IS), 20 and 21. Its course 

 is shown on the geological map by the brown color which marks the out- 

 crop of the Red Beds. 



Though it is easily distinguishable and traceable around all parts of the 

 Hills, preserving the same general features, it is subject to variations in 

 width and in details of structure. Its width depends upon the width of the 

 outcrop of the upper red clay and that depends on the local dip. It depends 

 also, but to a less extent, on the thickness of the clay. Where the dip is 

 slight, the outcrop and the valley are broad; where it is heavy they are 

 narrow, but there is no place where the valley is less than a mile across. 



The Indians, recognizing its continuity and the regularity of its surface, 

 have followed it with their great trails or routes of travel, and it is known 

 to them as the "Race-course." 



It is generally well covered with the common short grass of the Plains, 

 but it is entirely destitute of trees, save that an occasional hill may sustain 

 a few pines The immediate valleys of the streams and dry washes which 

 drain across it from the interior of the Hills are narrow and frequently lined 

 with small groves or scattered individual trees. Their principal tree is the 

 cottonwood, but there are occasionally dwarfed and stunted oaks and 

 thickets of willow and wild plum. As already remarked, the majority of 



