290 GEOLOGY OF THE BLACK HILLS. 



of the stream became insufficient to transport farther in large quantities 

 bowlders of this size and weight, and the beds resemble somewhat shore or 

 estuary deposits, such as are formed near the mouth of a stream emptying 

 into a lake or sea. These gravel banks have been formed since the eleva- 

 tion of the Black Hills, and the great Tertiary sea which gave rise to the 

 extensive beds of that age in the valleys of White River and the South 

 Cheyenne may have reached to the foothills of this range. A gravel 

 " wash" from the Black Hills is found all over the surface of the plains to 

 the north, east, and south of the range. Near the Cheyenne this gravel 

 forms a conglomerate 6 feet in thickness between the top of Cretaceous No. 

 5 and the base of the White River Tertiary, and also occurs as scattered 

 gravel and bowlders on the tops of the Tertiary hills, showing that it has 

 been deposited since the close of the Cretaceous period, both before and 

 after the later Tertiary. These gravel beds are made up of a mixture of 

 bowlders and pebbles from all the harder rocks found in the different geo- 

 logical formations on the eastern slope of the Black Hills, including granite, 

 trachyte, schist, slate, quartzite, and quartz in all its varieties, mixed with 

 a variable proportion of sandstone and limestone from the disintegration of 

 the Potsdam, Carboniferous, and Red Beds. 



The preponderance of any particular rocks in the gravel seems to be 

 the result of the position of the deposits and the circumstances of influencing 

 the erosion which produced it. The more elevated beds contain a less pro- 

 portion of limestone and sandstone bowlders, mingled with those from the 

 Archsean rocks, than the deposits at lower levels, formed after deep gorges 

 had been cut by the streams through the recent formations A layer of 

 gravel a few inches in thickness covered the lower foothills near Box Elder 

 Creek, but the deposits were thin and spread out over a considerable area. 

 It was impossible to prospect them for gold, as there was not a drop of 

 water in the bed of the stream for several miles up the cation. This pecu- 

 liar formation of high gravel-capped hills does not become extensively 

 developed north at the valley of Rapid Creek, although a thin wash of lime- 

 stone and sandstone bowlders, mingled with a little quartz and slate, covers 

 the foothills as far to the north and west as the forks of Spearfisk and 

 Redwater. 



