366 



GEORGE B. RICHARDSON 



DESCRIPTION. 



General description. — The accompanying map shows the 

 Spearfish formation girdling the Black hills in a crude ellipse, 

 one axis of which is about eighty miles and the other forty. 

 The width of outcrop varies considerably, but averages two miles. 

 The greatest areal extent is in the vicinity of Sundance, where 

 the formation is eight miles wide ; near Cascade Springs the 



Red Butte, northeast of Cambria, Wyoming. Spearfish red beds capped by 30- 

 foot bed of gypsum. 



width, because of the steep dip there, diminishes to less than a 

 thousand feet. 



Structurally these rocks conform to the contiguous sedimen- 

 tary formations by dipping on all sides from the central mass of 

 the Black hills. This general dome structure, however, is varied 

 by subsidiary folding. At the inner limit of the upper red beds, 

 toward the center of the hills, the underlying purple limestone 

 slopes upward, and the line of contact, where the softer red 



