128 GEOLOGY OF TDK BLACK HILLS. 



liood of Elk Creek and beyond, where the slates carry numerous outlying 

 l)atclies of Potsdam and Carboniferous, the cliff loses its regularity, but 

 there is still no difBculty in distinguishing the several beds. 



In the cafion of Elk Creek, which for several miles cuts a vertical and 

 tortuous channel 200 or 300 feet in depth througli the Carboniferous and 

 lower Red Beds, excellent sections are exposed of the Carboniferous rocks, 

 and they were examined at several points as our trail descended into or 

 wound its way out of the intricacies of the cailon. The cross-section 

 already given of the development and pitch of the Carboniferous on Rapid 

 Creek would answer almost equally well for a section on Elk Creek. The 

 alternating bands of colored limestone and sandstone of the upper series 

 are again well exposed, but the outcrops were not studied with any minute 

 care. 



The four creeks which cross the limestone ridge at the north end of its 

 bow-shaped curve and flow to the Belle Fourche were not followed through 

 their canons. The next point examined on the margin of the uplift was the 

 cailon of Spearfish Creek. The Carboniferous is well exposed, but owing 

 to the presence of several volcanic peaks it is much tilted. Near the mouth 

 of the canon the intrusion of the volcanic rocks of Black Butte on the south 

 side of the creek has raised and exposed the lower sedimentary rocks, and 

 through them the creek flows in a profound cafion. The following section 

 was taken at this place, beginning in the Red Valley. 



Red Beds. 



Feet. 



5. Purplish liuiestoue 25 



4. Cruiiibliug, soft, red sandstone 100 



Carboniferous. 



3. White, yellow, orange, and dark red, massive, soft sandstone; irregular in 



color, and weathering in fantastic shapes 250 



2. Talus 200 



1. Siliciou.s limestone to the bed of the creek — 



Near the butte, resting on the volcanic rock of the peak, fragments of 

 the metamorphosed Potsdam were seen, and resting upon it in regular order 

 are the strata of the above section. 



Around Crcnv Peak the Potsdam, altered to a hard quartzite, stands 

 nearly vertical resting against the trachyte. The Carboniferous also has 



