140 



GEOLOGY OF TDE BLACK HILLS. 



luulorlx'ing red sandstones wliich form Fanny Peak. The water of Beaver 

 Creek issuing- largely by springs from beneath the limestone is hard and 

 limy, and nuin^- of the tributaries as they enter the Red Valley and become 

 exposed to rapid evaporation deposit beds of travei'tina 



West. priMl East. 



5 4 4 3 2 1 



Fig. 19. — Ideal section across the Red Valley at Camp Jenney; sliowiDg the foothills at the left 



and Fanny Peak at tlic right. 



1. Carboniferous limestone. 4. Red clays with gypsum (Red Beds). 



2. Red sandstone (Carboniferous). r>. Jura. 



3. Purple limestone (Red Beds). C. Cretaceous. 



Camp Jenney is at the point where Beaver Creek leaves the Red Val- 

 ley and cuts through the Cretaceous rampart on its way to the Cheyenne. 

 To the southward the valley rises gradually in a prairie-like expanse, des- 

 titute of water and but little broken by the dry channels that traverse it. 

 Thence to Red Caiion Creek it was not examined geologically, but the 

 account given by the topographical party shows that there are no peculiar 

 features. 



On Red Canon Creek the Red Beds are finely exposed, and the width 

 of the valley, including the slope of the purple limestone, is fully eight 

 miles. Here again portions of the red clay are sandy, sometimes becoming 

 a sandy shale, and in several places a tolerably massive but soft, argillace- 

 ous red sandstone. This character, as well as the relation of the red clavs 

 to the overlying Jura, is illustrated in the following section taken near 

 the foothills on the west side of Red Canon Creek. 



Jura. 



Feet. 



5. White ripple-marked .sandstone, with a thin calcareous layer at its base full 

 of Jurassic fossils : EhynchoneUa myrina, Pseudomonotk curta, and Trape- 

 zium subequalis 20 



4. Gray shales or marls 15 



3. White sandstone, soft, and at bottom sbalj^, weathering in pot holes 15 



