298 GEOLOGY OF THE BLACK RILLS. 



on the Cheyenne, between the mouths of Rapid and Spring Creeks, where 

 this conglomerate, about 6 feet in thickness, caps hills of Ci'etaceous shales 

 500 feet in lieight, can the result of this erosion be identified. There the 

 conglomerate is made up of small bowlders, round and water-worn, of 

 granite, trachyte, slate, quartzite, and quartz, in all the vaneties in which it 

 is now found in the Black Hills, together with chert nodules from the Carbon- 

 iferous limestone, and fragments of hard quartzite from the Cretaceous. 



The third erosion occurred during the Glacial or Drift period, when 

 thin beds of gravel, with occasional Ijirge bowlders, were strewn over the 

 surface of the plains for a distance of fifty miles from the foothills, and 

 I'est equally on the surface of the diff'erent members of the Cretaceous and 

 cap hills of the White River Tertiary. Some of these traveled bowlders 

 are 2 feet in diameter, and are most abundant on the Cheyenne to the 

 southeast of the Black Hills. They have evidently been transported by 

 the agency of floating ice. Any current of water sufficiently powerful to 

 carry bowlders of tliis size and weight would entirely remove and sweep 

 away the soft clay-shales on which they rest. There was no evidence 

 found that during the Quaternary period any extensive glaciers occurred 

 in the Black Hills. Even the tops of the highest peaks showed no glacial 

 scratches wdiere the hardness of the rocks would insure their preservation. 

 A few large bowlders of granite, 8 to 10 feet in diameter, were observed 

 perched on the tops of rocky benches at the bends of the small streams flow- 

 ing southeast from Harney Peak. They resemble those referred to glacial 

 action in the Eastern States, but were possibly only the result of a peculiar 

 weathering of the granite. 



While there is little evidence of the presence of extensive glaciers in 

 the Black Hills, the occurrence of large quantities of ice and snow in this 

 elevated region during the Glacial period, with the necessary accompani- 

 ment of increased rainfall and river action, have probably been the agents 

 which have caused this immense denudation of the sedimentary strata. And 

 it seems almost necessary to assume the occurrence of an extensi^■e lake sur- 

 rounding the Hills during the Quaternary period, when the bowlders result- 

 ing from tlie erosion were transported by the agency of the floating ice to 

 the places where they are now found. 



