244 GEOLOGY OF ASPEN MINING DISTRICT, COLORADO. 
of the shales and porphyry which are contained in the Aspen Mountain 
syncline. Erosion of shales under the same conditions has formed the 
shallow basin of Tourtelotte Park. 
INFLUENCE OF FAULTS. 
The faults also, as well as the folds, have greatly influenced the topog- 
raphy. Gulches following the outcrops of faults are very common—tor 
example, the upper part of Queens and Spar gulches, and Copper Gulch. » 
The reason for the formation of gulches along fault lines appears to lie 
partly in the fact that fault zones are in brecciated material, which is more 
easily eroded than the surrounding rocks. It is notable, however, that in 
Spar Gulch and Queens Gulch, only the upper part of the gulch lies in the 
fault, the lower part diverging from it to jom some larger channel. These 
fault zones are channels of underground water, which occasionally rises to 
the surface as springs, and in Queens Gulch such a spring, which is really 
a small underground stream, rises along the Castle Creek fault, and it is 
from the erosion of this water that the gulch appears to have been chiefly 
formed. 
GLACIATION. 
Most of the topographical forms have been influenced by glacial action. 
Glacial drift is found over the whole district, although often locally stripped 
off by more recent erosion. In the river beds the glacial deposits are 
somewhat modified, and this rearranged drift forms the floor of the larger 
valleys, such as Roaring Fork and Hunter Creek. On the high hills, 
however, the drift exists as unmodified moraine. On Ajax Hill, which lies 
between Roaring Fork and Castle Creek, and above Aspen Mountain and 
Tourtelotte Park, there is found all along the southwest side, near the 
summit, a broad bench, of glacial origin, carved in the bed rock. This 
bench is ordinarily covered with morainal material, so that there are no 
outcrops. The material consists of a granitic matrix containing quartzite 
and granite bowlders, none of which are of very great dimensions, and is 
comparatively uniform, even when the underlying rock is dolomite and 
limestone. At intervals, however, this bench has been cut through by 
post-Glacial erosion, and the covering of drift has been stripped away. 
Thus strongly marked gulches are formed with bare rock on both sides. 
Toward the south the bench widens out, and the topography of the whole 
