60 GEOLOGICAL SURVEY OF THE TERRITORIES. 



geologic condition. The depression is by no means uniform, and the 

 movements of the stream-bed are quite eccentric, making altogether a 

 very interesting study. 



From the mouth of Maroon Creek to the mouth of Sopris Creek, a 

 distance of some twelve miles, there is a pretty well marked fault, not 

 following the line of greatest depression, but occurring along the eastern 

 slope from one-fourth to three-fourths of a mile from the axis of the 

 fold. We thus have a fracture parallel with a fold, and the two lines 

 seem to contend for the privilege of accommodating the stream-bed. 

 Above Maroon Creek (see map and sections) the river flows in the syn- 

 clinal ; near the mouth of the same creek, it is in the fault. Below this 

 it cuts through the beds again and follows the sjnclinal for a number of 

 miles. Still lower it turns again to the right, into the fault, and follows 

 it all along the eastern base of the isolated ridge mentioned at the out- 

 set. Leaving this again below the mouth of Sopris Creek, it continues 

 in the fold, while the fault probably dies away. The dislocation, if any, 

 of the beds, as exposed on opposite sides of the stream, is so slight that 

 it seems quite impossible to determine this point. The downthrow is 

 generally on the west, and does not amount in any case to more than 

 3,000 feet. In two localities along the fault, there have been outflows 

 of lava. These were observed by Dr. Peale, in 1873, and are located, 

 the lower one, opposite the mouth of Sopris Creek, where it caps a large 

 rounded butte, (see general map.) The other is on the same side of the 

 river, some five miles farther up. The lava appears to be basaltic, and 

 has quantities of cinder and ashes associated with it. It caps an im- 

 portant butte near the river-bank, forms an escarpment some 100 feet in 

 height, and covers an area of scarcely more than half a square mile. 

 Section C of the large sheet cuts this butte, and shows at the same 

 time a most remarkable displacement, the edges of the strata on both 

 sides of the fault being turned abruptly up, and therefore dipping 

 from the plane of the fault, both toward the upthrow and the down- 

 throw. The beds on the west side are depressed so that the Lower 

 Cretaceous rocks seem to face the Upper Carboniferous of the east side. 

 The upturned edges were apparently leveled off before the flow of 

 lava took place. At the mouth of Maroon Creek the depression of 

 the west side has been much greater, and the edges of the beds have 

 been dragged upward and, apparently by a lateral movement, forced 

 past the vertical. Thus is formed the little butte of Cretaceous and 

 Jurassic rocks between Maroon Creek and Eoaring Fork, at the junc- 

 tion described by Dr. Peale, (Eeport for 1873, page 263.) Shortly above 

 this the fault becomes a fold and so continues up the valley of Castle 

 Creek. 



The sections of the accompanying x)late. Fig. 1, cut ten of the most in- 

 teresting points along the line of disturbance, and is intended to give a 

 connected idea of the folding and dislocations. The sections are so placed 

 as to give the impression that perspective is taken into account and that 

 the point of view is somewhere on the lower course of Roaring Fork. It 

 ■will be observed, by reference to these, that the southern extension of 

 the synclinal follows the valley of Castle Creek, and that the upper 

 course of Eoaring Fork proper is in the granite to the east. A still 

 more extended examination to the southward and beyond the sources of 

 Castle Creek, seams to warrant the conclusion that the portions of Silu- 

 rian (?) quartzite noticed on the east face of station 3 and along the 

 summit of the Italian group beyond, indicate a continuation of the same 

 fold or at least of the same movements that produced the fold. That 

 this is the case, and, therefore, that the entire geologic phenomena of 



