104 3EOLOGY OF ASPEN MINING DISTRICT, COLORADO. 
Pl. VII gives a view of the perpendicular cliff of jointed Silurian dolomite, 
with its polished and striated face. 
Another fault, which appears to be post-Glacial and which belongs to 
the same system, is the Burro. Where this fault crosses the ridge to the 
west of Spar Gulch there is a marked north-facing escarpment, approxi- 
mately 150 feet high, which is about the actual throw of the rocks at this ° 
point. Pl. VIII is a view taken from the west side of Castle Creek fault, 
looking eastward across the intervening valley. The escarpment is seen at 
the top of the picture against the horizon. If this is imagimed to be removed 
and the right half of the hill depressed to a level with the left half, it will 
be seen that the hill has the lenticular, drumlinoid outline which is so char- 
acteristic of glaciated surfaces. There can be little doubt that this was the 
actual form in which it was left by the overriding glaciers. The preserva- 
tion of this smooth outline to the present day shows that there has been no 
great amount of post-Glacial erosion; and since the amount by which this 
typical drumlinoid outline has been displaced is exactly equal in amount 
and direction to the displacement of the underlying rocks by the Burro 
fault, it seems highly probable that the whole movement has come about 
in post-Glacial times. The rocks which now outcrop on the hill are, on 
the right side of the escarpment, porphyry, and on the left the blue Weber 
limestones which underlie the porphyry. — 
The Butte and Burro faults are probably the most interesting and 
conclusive proofs of the recent age of some faults of the east-west system. 
There is, however, considerable additional evidence in various places to 
show that the movements along many of these faults are still going on, and 
some of them are probably attaining their maximum development at the 
present time. Along many of the faults in Tourtelotte Park there are 
scarps, but it must be borne in mind that such a scarp is not necessarily 
one of uplift, but may be a scarp of erosion. Such an erosion scarp may 
occur along a fault which has not had great development since Glacial 
times, and may result from the rapid wearing away of soft beds on one 
side of the fault as compared with the slower degradation of the harder 
beds on the other. In PL IX is shown such an erosion scarp, which was 
formed in Glacial time. This is along the Sarah Jane fault. The plate 
looks to the northwest. On the right side the surface is underlain by the 
soft shales of the Weber, while on the hill to the left the blue Leadville 
