32 GEOLOGY OF THE BLACK RILLS. 



10. Soft incoherent white sandstone, with bands of harder red sandstone and 



small feiTutfinous concretions (feet) . . 135 



1). Li finite Thin. 



(On tin; sonth side of the river found with its under clay to be 4 feet thick.) 



8. Yellowish and reddish sandstone (feet) . . 8 



7. Baud of iron-ore nodules (inches) . . 3-10 



(J. Soft yellow ish sandstone, with iron nodules (feet) . . 22 



5. Lignite . Thin. 



4. Brown sand}' clay, with much carboniferous matter and iron nodules; with 



fragments of plant impressions (feet) . . 10 



3. Lignite (inches) . . 8-10 



2. Dark fire-clay (feet) . . 5 



1, Soft whitish sandstone to bed of the Cheyenne River (feet) . . 15 



Total height of section (feet) . . 250 



These beds are considered to belong to the Fort Union group or Great 

 Lignitic Tertiary, wliich contains such large beds of lignite in the Upper 

 Missouri region. In this locality the beds have a very slight dip toward 

 the west, about 4° or 5°, which is the same as the angle observed in the 

 beds of No. 5 Cretaceous exposed a few miles farther east. In other 

 portions of the northwest the series of Tertiary beds is found resting con- 

 formably upon the upper Cretaceous and underlying tlie deposits of the 

 White River Tertiaries. 



From this point on the Cheyenne River northwestward to the Black 

 Hills one rises gradually to their elevation over a succession of monoclinal 

 ridges or hog-backs of Cretaceous rocks, which wnth more or less regulanty 

 are found in parallel and continuous lines surrounding the Hills. The 

 strata composing them dip from the Hills at varying but low angles — 

 5° to 10° or 25°. They are monoclinal ridges of denudation, the remnants 

 of strata which formerly were continuous across the area now occupied 

 by the Hills; and across their escarped edges, facing the Hills, one passes 

 from the highest to the lowest strata of the Cretaceous. Definition has been 

 given to the ridges by the interposition among softer beds of certain more 

 resistant strata. The most prominent and persistent of these occasions the 

 range of rampart-like foothills that surrounds the immediate base of the 

 mountains, forming the outer margin of the remarkable Red Valley, w Inch 

 as a moat encircles the Hills. This inner and last of the series of ridges 



