11(3 GEOLOGY OF THE BLACK HILLS. 



the sui-fjico has been destroyed by the numerous forks and. branches of 

 Ii)}an Kara, Redwater, and Spearftsli Creeks, which have cut canons in 

 all directions often to the depth of several hundred feet. Still a consider- 

 able share of the orij^inal surface survives, and it is easy to construct from 

 the many tongues and spurs that remain a clear idea of the orif^inal form. 

 If one views it from a station a little elevated above it, he sees clearly the 

 plateau character, but may fail to comprehend how great a task hes before 

 him should he attempt to force a passage across it. 



The exposures of the Carboniferous on the eastern margin of the 

 uj)lift are comparatively small in extent, and consist entirely of rocks 

 dipping rapidly to the east. They constitute a monoclinal ridge curving 

 about the edjje of the Archa^^an. The creeks which drain the Archaean flow 

 to the north, east, and south, and cut through this rim of Carboniferous 

 rocks in canons whose length depends on the rate of dip of the strata. 



In the Bear Lod^e ranjje the Carboniferous has been tilted and raised 

 to view, along with the other strata, by the elevation that accompanied the 

 volcanic outbursts of Warren Peaks. The outcrop is not large and the 

 strata composing it hardly differ from those in the main portion of the Hills 

 save in a slight amount of metamorphisni due to the volcanism Their dip 

 is outward from the peaks in all directions, but most strongly towards the 

 northwest. 



The details of the geological structure of the Carboniferous mesa north 

 of Crook Tower were not studied with much care for lack of the time neces- 

 sary to penetrate its labyrinth of canons. The dip of the rocks where 

 the}' curve down from the table toward the Red Valley is more gradual 

 than at the south and southwest of Castle Creek. Crossing it in a north- 

 west direction we found the surface slope, canons excepted, rarely greater 

 than 6° or 8° ; and from well-marked Carboniferous rocks one passes 

 almost imperceptibl}- to the Red Bed limestone, which continues the slope 

 more steeply until it dips under the mass of red clays underlying the great 

 Red Bed valley that sweeps around the northwestern end of the Hills. 

 Because of the gentleness of this slope and the ready manner in which 

 the up])er Carboniferous arenaceous strata decompose and form a soil, but 

 little is seen of them in passing over the slopes, though excellent exposures 



