CHAPTER V. 
SURFACE CHANGES SINCE ORE DEPOSITION. 
AMOUNT OF EROSION. 
Since the beginning of the period of deformation in the Aspen district, 
which gave rise to various physical and chemical changes, of which ore 
deposition is among the most interesting, an enormous amount of erosion 
has taken place. Previous to the Cretaceous uplift there extended over 
the whole of this district a thickness of at least 15,000 feet of sediments 
overlying the granite. That this is true is shown by the fact that these 
sediments still actually exist in that part of the district which hes west of 
the Castle Creek fault. East of the fault, however, in the metalliferous 
district, the rate of uplifting was vastly greater than farther west, bringing 
the granite and the lower sedimentary beds to the position they now occupy 
on Aspen Mountain and Tourtelotte Park; and the relief resulting from this 
uplift caused accelerated erosion, which from that time to the present has 
removed the sedimentary beds down to the granite, a thickness, as stated, 
of over 15,000 feet. 
DIFFERENTIAL EROSION. 
Most of the topographical features of the district have been largely 
influenced in their formation by the structure of the underlying rock. Thus 
it is noticeable everywhere that erosion of shales and sandstones has been 
greater than that of the more resistant granite, quartzite, and solid lime- 
stones. For this reason the valleys of Roaring Fork and Woody Creek 
widen immediately after emerging from the granite upon the softer sedimen- 
tary beds; and Aspen Mountain, where an uplift has produced a peculiar 
underground structure differing from the rest of the district, stands out also 
as a peculiar topographical form, the ridges of East Aspen and West Aspen 
.mountains being formed by the resistant granite, quartzite, and dolomite, 
and the broad depression between them resulting from the greater erosion 
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