150 GEOLOGY OF ASPEN MINING DISTRICT, COLORADO. 
and Red mountains. After the disappearance of this vast ice sheet there 
was a comparatively quiet period, during which erosion went on rapidly and 
the river valleys were filled with local glaciers, which formed distinct and 
later moraines. In many cases the débris of this earlier ice sheet has been 
stripped by erosion from the mountain sides and is found only on top, as 
in the case of Red Mountain. In other places it has probably been entirely 
remoyed. This amount of erosion, even taking into consideration the actiy- 
ity with which material was removed in this mountainous region, indicates 
that the main ice sheet disappeared at no very recent period. We may 
therefore assume that the minimum figure given by Upham represents the 
period of post-Glacial time in the Rocky Mountains, namely, six thousand 
years. If the Butte fault has moved 400 feet in that time this would make 
arate of about 1 foot in fifteen years. This rate is the maximum, so far as 
is actually known, of present movement, and in estimating the rate of the 
faulting as a whole other considerations come in. At the beginning of the 
uplift and faulting there was a great load of overlying rocks in the uplifted 
areas and this load rendered faulting slower and more difficult. The load 
amounted to at least 15,000 feet of strata on the east side of the fault at 
Tourtelotte Park. These 15,000 feet are all exposed in the district just 
west of the fault, but on the east side are stripped away by erosion, which 
has usually kept pace with the uplifting and faulting, though often lagging 
behind. The disturbance which began under this heavy load consisted in 
the upbending of the strata, with few fractures, but along these few the dis- 
placement was important. With the stripping of the strata there developed 
more numerous faults, which, however, had in general slighter movement. 
Thus the intersecting faults in Tourtelotte Park are numerous and compli- 
cated, but are all comparatively late in origin. So the rate of the fault 
movement at the present day, which has been approximated at a maximum 
of 1 foot in fifteen years, is probably the maximum for the whole period of 
deformation. It has been roughly estimated that about two-thirds of the 
faults originated since the ore deposition, but the premineral faults are 
characteristically heavier, and from the considerations above stated it seems 
probable that the ore deposition lasted through two-thirds or even more of 
the time from the beginning of the uplift to the present day. Thus the ore 
began to form under not more than 15,000 feet of sediments and probably 
ceased when covered with 5,000 feet or less. 
