524 



E. RUSSELL LLOYD AND C. J. HARES 



Indian Reservation/ in both of which one of the writers (Lloyd) 

 has been able to distinguish the upper marine member. Parts 

 of these areas are included in the accompanying map (Fig. i). 



Throughout the greater part of the area studied by the writers 

 the strata are nearly flat-lying, with only a very low general dip, 

 usually less than 30 feet per mile, to the north or northeast. Prac- 

 tically the entire area is grass-covered and only in the bluffs of 

 the larger streams and locally in high steep-sided buttes are natural 

 rock exposures found. The relief is very slight; hence there is 

 considerable difficulty in tracing the formation boundaries. 



The general character and relations of the Cretaceous and 

 Tertiary formations exposed in this part of the Dakotas are shown 

 in Table 1. The areal distribution is shown on the map (Fig. i). 



TABLE I 



Cretaceous and Tertiary Formations in Western North and South Dakota 



System 



Se- 

 ries 



Formation 



Character 



Thickness in 

 Feet 



Tertiary 



6 aJ 



^ 



White River 



Cross-bedded sandstone and fresh- 

 water limestone 



140 



Unconformity, angular up to 20° and erosional up to 1,500 feet 



Tertiary 



rt 



1^ 



Fort Union 



Yellow sandstone, shale, clay, and 

 lignite 



1,025 



Tertiary ? 



n-. 



<u 

 C 

 1) 

 <J 

 



W 



Lance 



Cannonball marine member. Dark 

 sandy shale, dark shaly sandstone, 

 and yellow sandstone, containing 

 marine shells 



Ludlow lignitic member. Light san- 

 dy shale, calcareous sandstone and 

 lignite 



Somber-colored shale, yellow sand- 

 stone, and thin lignite beds 



0-300 



0-350 

 400-525 



Cretaceous 



Oh Oj 



Fox Hills 



Sandstone, yellow 



25-400 



Pierre 



Dark marine shale 



300-1- 



' W. R. Calvert and Others, "Geology of the Standing Rock and Cheyenne 

 River Indian Reservations, North and South Dakota," U.S. Geol. Survey Bull, jyj, 

 1914. 



