368 GEORGE B. RICHARDSON 



rocks have been eroded from the surface of the limestone, is 

 extremely irregular. The outer limit of the formation is 

 emphasized by an escarpment formed by Jurassic and Cretaceous 

 sandstones. 



Streams coming from the high central area of the Black hills 

 generally flow directly across the red beds, yet occasional creeks, 

 like Red Water in the vicinity of Beulah, flow for some distance 

 parallel to the strike of the red rocks before leaving them. The 

 divides, however, are low and the general appearance of the 

 Spearfish formation is that of a valley. This red valley is very 

 conspicuous. No timber and very little vegetation are supported 

 by the underlying rocks. Erosion has cut gullies in the shales 

 and has exposed prominent gypsum-capped buttes, while the 

 color contrasts of the rich red rocks and pure white gypsum form 

 striking effects. 



Details of stratigraphy. — The stratigraphy of the Spearfish for- 

 mation is shown in detail by the following: 



COLUMNAR SECTIONS. 

 (The heavy lines mark respectively the upper and lower contacts of the Spearfish red beds.) 



I. ONE MILE UP COLD BROOK FROM HOT SPRINGS. 



Feet. Inches. 



Upper contact covered. 



Gypsum - - - - - - - - - - -lo 



Red sandy shale .-.-.---- 3 



Massive white gypsum .---.-.. 3 



Fine red shale - - - - - - - - -'15 



White gypsum ----------.. g 



Red clayey sandstone -------- 8 



White gypsum --.-------.. 8 



Red clayey sandstone - - - - - - - - 15 



Massive mottled gypsum (red sandy clay admixture) - - - 35 



Red sandy shale with interlacing veins of gypsum i inch to -1^^ inch 



thick --..------ 3 



Massive white gypsum -------- 3 



Green-drab shale --------- i 3 



Chocolate-brown hackley shale -------5 



White gypsum -..--...- 4 



Hard light-red clayey sandstone - - - - - - . . 8 



Green shale -.--------.. )^ 



Hackly brown sandy shale with a 3-inch streak of green clay - 15 



