178 GEOLOGY OF THE BLACK HILLS. 



character of the Dakota outcrop is distinctly marked, and it is clearly seen 

 to merge at the east and west into the south-trending" "foothill" ridges. 



If the reader will refer to the geological map and the bird's-eye view 

 in connection with this description, he will obtain a clearer idea of the 

 topography of the sandstone. 



Excepting in the very extremities of the uplift the streams which cross 

 the Dakota outcrop pass from the central region of the Hills outward toward 

 the Plains. All of the streams, perennial or occasional, that flow from the 

 limestone region into the Red Valley, escape from the valley by breaking 

 through the foothill ridge — either singly or in combination — and the result 

 is a series of cross-cutting canons in which the composition of the Dakota 

 group can readily be examined. A few of the succeeding pages will be 

 devoted to the enumeration of some of the details thus ascertained. 



At the Beaver Creek gap the summit of the cliff overlying the Jura 

 is formed of — 



Feet. 



1. Sandstone, at base very soft and white, but for the upper 50 feet yellowish 



and coarse 150 



This sandstone dips westward 20° to 25°, and near the foot of its slope is a 

 second low ridge containing at its base, resting upon the above sandstone — 



2. Sandstone, whitish and yellowish, thinly bedded, and with many ripple marks . 60 



3. Sandy shales 20 



4. Sandstone, gray and reddish ; in places thinly bedded; with ripple marts. . . 64 



5. Gray clay or marl, with nodules of impure limestone, strata of siderite, much 



cone-in-cone, and selenite 20 



6. Sandstone, with carbonaceous matter 10 



7. Gray shale and yellow clay • 15 



8. Sandstone 15 



No fossils were found in these rocks, but they are all referred to the 

 Dakota group because they are immediately underlaid by Jurassic strata, 

 well characterized by fossils, and they immediately underlie the black plastic 

 clays of the Fort Benton group. 



The ridge character of the outcrop continues southward to the gap by 

 which Red Canon Creek issues from Red Valley, and there ceases. The 

 southern plateau is included between Red Canon and Minne Katta Creeks. 

 The next points of examination were on and near the plateau. A section 

 of the cliff at Red Canon Creek, to be found in the Jurassic division of the 



