118 GEOLOGY OF THE BLACK HILLS. 



fish has cut a canon from 300 to 600 feet deep, often penetrating into and 

 below the underlying Potsdam. Frequent exposures of the variegated 

 sandstones of the upper member of the Carboniferous are seen in the west- 

 ern valleys, where it is a prominent feature, but the intermediate series of 

 alternating sandstones and limestones were not observed. It is not con- 

 sidered that they are here absent, but merely that their outcrop is locally 

 concealed ; for they are found farther north on lower Redwater Creek, 

 farther south in some of the canons running into Beaver Creek, and on the 

 eastern hog-back wherever the Carboniferous was carefully examined. 



The eastern margin of the plateau where it overlooks the country 

 around Custer and Terry Peaks has much the same character as farther 

 south, but it is flanked by more numerous outliers or mesas of Carbonifer- 

 ous and Potsdam. Northeast of Terry Peak, on the ridges between the 

 creeks, a series of mesas extends across the slate area. 



The plateau south of Crook Tower, so far as examined, is less cut by 

 the canons of draining streams. From an elevation near Castle Creek of 

 7,000 feet or a little more it dips off to the south and southwest, until 

 at the head of Red Canon Creek it is 1,000 feet lower, and thence its 

 surface soon joins the Red Valley on the south. The extreme southern 

 end was passed over by some members of the escort, who reported its sur- 

 face as deeply scored by the canons of streams draining into Beaver Creek 

 and the Cheyenne, in which character it resembles the extreme northern 

 end of the plateau. Along the eastern margin of the table near Castle 

 Creek there is a small southward dip and it increases toward the southern 

 end. There is also a general dip to the westward, and this, while barely 

 discernible on the eastern side, increases rapidly toward the west and 

 southwest, until at last the Carboniferous rocks dip steeply under the Red 

 Valley. 



At the western margin of the plateau near Camp Jenney, and for some 

 distance along the east fork of the Beaver, there is a remarkable and ex- 

 ceptional dip, which is exceeded only in the rocks immediately surrounding 

 some of the volcanic peaks at the north. It is illustrated in the accom- 

 panying section, running from the Red Valley at Camp Jenney eastward 

 across the mesa to the headwaters of French Creek. 



