412 GEOLOGICAL EXCURSION TO THE ROCKY MOUNTAINS. 



ful summit of Sopris. which is most beautiful in the spring and autumn 

 when it is freshly covered with snow. 



For about 12 miles (1!) km.) above Rock Creek the valley bottom is 

 still in soft Upper Carboniferous beds, somewhat faulted and contorted 

 but in general occupying a gentle anticline between the synclinal fold 

 on the west, at the east base of Sopris, and the triangular syncline to 

 the eastward already mentioned. The latter has been considerably 

 eroded and subsequently covered with basalt flows, parts of which still 

 cover the higher flat-topped ridges, and their debris may be distin- 

 guished on the east side of the valley, mingled with that of the red 

 sandstones which form the bounding cliffs. 



On the west side of the valley the lighter-colored Jurassic beds may 

 be distinguished above the red sandstones, which now come down to 

 the valley bottom with a westerly dip. 



At Aspen Junction the main line of the Colorado Midland leaves 

 the Roaring Fork valley and follows up that of Frying Pan creek in 

 an easterly direction. For many miles above the junction this line 

 follows a winding gorge cut in red sandstones of Triassic and Upper 

 Carboniferous age, which present grand oliff exposures. Beyond these 

 itpasses along more open valleys cut in Lower Paleozoic limestones and 

 (piart/ites, and tinally reaches the Archean granite and gneiss which 

 forms the core of the Sawatch range. A long tunnel in these rocks, 

 under the crest of the range, is mainly in granite. 



In the Roaring Pork valley, above Aspen Junction, the railroad 

 follows a ilood-plain bench through a widening of the valley, and then 

 passes through narrows formed by red sandstones, massive below, 

 thin bedded and lighter-colored above. The structure is here some- 

 what complicated, showing sharp folds and some faulting. In general 

 the route now lies along the eastern side of a synclinal in Cretaceous 

 and Jurassic beds, the northern continuation of the synclinal fold, 

 already mentioned as formed by t lie uplift around Sopris Peak. The 

 beds are mostly argillaceous shales with some impure limestones. On the 

 east side of a second widening of the valley, which shows consider- 

 able flood-plain terraces, is the mouth of Woody creek, another tribu- 

 tary draining the west slopes of the Sawatch. 



The road now passes along the strike of the westerly dipping Cre- 

 taceous shales and limestones, the hills on the east being formed of 

 underlying red sandstones, while on the west the sharp Synclinal fold 

 can be distinguished in a harder limestone bed (probably Niobrara 

 Cretaceous). 



Just before reaching Aspen a flood-plain terrace is crossed between 

 Maroon and Castle creeks, up whose valleys, to the west and north 

 respectively, distant glimpses may be had of the castellated forms 

 around Maroon and Pyramid peaks in the Elk Mountains. 



The valley gorge of Maroon creek is cut, in an east and west direc- 



