CHAPTER VI. 



BEPOET O^ THE GEOLOGY OF THE IsOETHWESTERN POE- 

 TlOiS^ OF THE ELK EANGE. 



By W. H. Holmes. 



Deae, Sm : In accordance with instructions received from you, I con- 

 tinued the geologic examination of the northwest portion of the Elk 

 Mountains, and beg leave to present the following report: 



When it was found that, on account of the sickness of Mr. Shanks, 

 assistant topographer, the main party could not advance beyond Capi- 

 tol Creek, a small party, consisting of Mr. George B. Chittenden, 

 topographer, myself, and one packer, was detailed to continue the survey 

 around to the northwest. 



It was arranged that we should carry with us xjrovisions for fifteen 

 days, and that a supplj'-party should meet us on the western side of the 

 range, near the sources of East Eiver, if the main party should not be 

 able to reach that point in time. 



On the 29th day of August, we left the main camp and moved down 

 the valley of Eoaring Fork. A well-marked Indian trail led us through 

 a low, synclinal depression, which is separated from the channel occu- 

 pied by the river, by a long, narrow ridge or hog-back. The depression 

 is occupied by the Lower Cretaceous shales, and the sandstones of the 

 Dakota group form the crest of the ridge. 



We soon crossed a low divide and were upon the southern branch of 

 Sopris Creek. This stream heads near the summit of the Elk range, 

 midway between Capitol and Sopris peaks, and on re9,ching the deepest 

 part of the depression, turns abruptly to the northwest and cuts ob- 

 liquel J" down through the ridge of Cretaceous, Jurassic, and Triassic rocks. 



The main Sopris Creek is formed in the snow-filled amphitheaters 

 about the eastern faces of Sopris peak and descends to the valley with 

 great rapidity, falling 6,000 feet in less than eight miles. It cuts its 

 way out from the granite to the Cretaceous rocks and then descends 

 with the dip of the beds, flowing for some time upon the hard floor of the 

 Dakota sandstones. The bed of the creek is everywhere very shallow, 

 and I was unable to determine whether any rocks lower than the Ju- 

 rassic were exposed or not. Dr. Peale, who climbed Sopris peak from 

 this side in 1873, states that there is but little exposure of the sedimen- 

 tary rocks along the north and northeast faces of the mountain, oij ac- 

 count of the great quantities of debris and morainal drift. 



THE EOAEINa FOEK SYNCLINAL. 



Before passing on to the description of Eock Creek and the western 

 slope, I wish to take a hasty review of the general geology of the valley 

 of Eoaring Fork, and give, if possible, a connected idea of its structure. 

 This valley is throughout, so far as examined, in the trough of a sj^n- 

 clinal fold, and its entire conformation, the course and form of the main 

 valley as well as of all its tribntaries, is undoubtedly the result of this 



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