ASPEN SPECIAL MAP. 5D 
gator. Through the western part of the area there runs in a nearly north- 
south line the great Castle Creek fault, which brings the Triassic beds on 
the west against the Maroon on the east, on the extremity of West Aspen 
Mountain; and farther south, as the throw of the fault increases, the Archean 
granite comes finally to lie against the Triassic. The narrow strip which 
lies between the Castle Creek fault and the west side of the area mapped 
consists chiefly of red beds of the Maroon and Triassic. The latter pre- 
dominates, but in Keno Gulch is the contact of Maroon and Triassic, 
which here, as elsewhere, is marked by a change from thin-bedded, shaly, 
and caleareous brown sandstones to the bright-colored, more massive, and 
purer Triassic sandstones. Throughout the whole of this strip the beds 
are overturned, so that the Maroon beds overlie the Triassic at their pomt 
of contact. All of these overturned beds form part of the closely com- 
pressed, easterly dipping, and northerly pitching syneline, which lies 
immediately west of the Castle Creek fault. 
Plate I (opposite p. 1) gives a general view of the area shown on the 
Aspen special map. The view is taken from a point down the Roaring 
Fork Valley at some little distance beyond the western edge of the map. 
In the foreground of this picture is the broad, flat, sage-covered valley 
bottom, made up of morainal material, which has been worked over by 
water action. Through this plain the Roaring Fork and its tributary 
streams flow, having carved out gorges of comparatively slight depth m the 
drift. At the right of the picture is Aspen Mountain, with its two prominent 
ridges separated by the broad, northerly facing depression. The nearest 
of these ridges is West Aspen Mountain; the farthest is Hast Aspen Moun- 
tain. The flat depression between the two occupies practically the same 
area as does an underlying synclinal fold in the rocks. The productive por- 
tion of Aspen Mountain lies almost wholly in this broad depression, although 
recently considerable ore has been discovered on the very pomt of West 
Aspen Mountain. In the central part of the picture the hills on both sides 
of Roaring Fork Valley appear to come together, and there is actually a 
great narrowing in the valley, caused by change of formation. West of this 
narrow point there lie sedimentary rocks, which, as in the case of the Weber 
and the Maroon formations, are often soft, and here a broader valley has 
been eroded: at the point of narrowing, however, granite comes in on both 
sides. It is probable that the broad valley below this granite gateway was 
