PEALEl GEOLOGY ROARING FORK. 87 



the creek there are exposures of Cretaceous beds in patches, whose re- 

 lations I was unable to determine definitely. 



Between the head of the creek and Frying-Pan Creek is a broad- 

 topped hill or mesa, capped with black vesicular basalt. 1 referred to 

 this mesa last year,* and then supposed the capping to be trachytic. 

 The beds beneath it, outcropping on the south and southeast sides, are 

 almost horizontal, the sandstone of the Dakota group (Cretaceous No. 1) 

 appearing on top, the Jurassic beds and Triassic sandstones lying be- 

 neath in their order. A short distance farther north, on Frying-Pan 

 Creek, the dip of the red sandstones, which outcrop in massive beds, is 

 a little east of north at a very slight angle, 5° to 10°. As we have 

 already seen, the Cretaceous formations'' on station 11 dip southwest. 

 The head of the creek that we are describing, probably has its origin in 

 a synclinal depression, which deepens to the northwest in going down 

 the creek, and gradually dies out beyond its head on the broad-topped 

 hills north of Frying-Pan. The folds in this region are generally very 

 gentle, but their axes run in almost every direction. There is so much 

 eruptive material on top of the sedimentary beds that it is difficult to 

 trace the conuections between the different outcrops. Mr. Marviue 

 thinks there is a fault running beneath the plateau, between the Grand 

 and Itoaring Fork. 



Scattered along the course of the creek, and its branches rising in 

 the plateau, are numerous little meadows. The lower seven miles of 

 its course the creek is in caiion, which deepens rapidly as we go down. 

 Therocks at the head are basaltic, capping the bluffs on either side. They 

 are present on the hills, or rather plateau, throughout the length of the 

 caiion. At one point 1 think Cretaceous shows, although I cannot be 

 certain, as I did not have time to visit the outcrop. Farther down, the 

 Eed Beds show, and beneath them, at the mouth of the creek, there is a 

 considerable thickness of the gypsiferous series. 



I will take up next the valley of Roaring Fork. The upper portion 

 of the valley was described in last year's report, so that this year we 

 have to do only with the lower part of its course, that below the mouth 

 of Frying-Pan Creek. There is one point, however, that I wish to refer 

 to here. When speaking t of the small butte between Maroon Creek 

 and Eoariug Fork, I was at a loss to account for the inversion of the 

 beds exposed in the butte of which I gave a section. This year 

 the Elk Mountain region was studied in more detail and a great many 

 obscure points were explained. In the case mentioned above, Mr. Holmes 

 found a line of faulting extending along the upper side of Roaring Fork 

 which explained the inversion of the strata. 



Below the mouth of Frying-Pan Creek on the right-hand side are low, 

 rolling hills, the basis of which seems to be the Upper Cretaceous forma- 

 tion. On the opposite side, however, there are outcrops of red sand- 

 stones beneath in tl^e bluffs. The Cretaceous strata extend to the gran- 

 ite of Sojjris peak, seeming to rest immediately upon it, there being 

 nothing showing between until we get on the other faces of the moun- 

 tain. The geology about the southern part of the peak was referred to 

 last year. Jurassic, Triassic, and Carboniferous layers outcrop on Rock 

 Creek, but this region will be fully described in other parts of the re- 

 port, so 1 merely mention it here and to return Roaring Fork. 



The course of the latter stream from the month of Frying-Pan to the 

 mouth of Rock Creek, a distance of eleven or twelve miles, is north 70*^ 

 west, the rate of fall being about fifty feet to the mile. The valley above 



*Page 265, report of ]873. tPage 268, Report 1873. 



