144 GEOLOGY OF ASPEN MINING DISTRICT, COLORADO. 
zone there was heavy folding, which resulted in the overthrust, easterly 
dipping, and northerly pitching folds of the Aspen district. These folds 
appear to have opened out to the north, and, as at present exposed in 
outcrop, are also open to the south. It is possible, however, that the 
compression was more intense in the upper sedimentary beds than in the 
lower ones, and therefore that those portions of the folds which are now 
open, in the southern part of the area, may have been closed and over- 
turned in the overlying sedimentary beds which have been removed by 
erosion. Between this line of greatest folding and the granite there was a 
series of slight open folds, which often are hardly recognizable and again 
are very noticeable. West of the line of folding there is very little defor- 
mation, the disturbance dying out in a surprisingly short space, so that the 
beds resume a horizontal or gently dipping attitude. 
The folding along this line culminated in a fault which appears to 
have originated along the axis of an overthrust anticline, and to have 
extended to the south and to the north along the line of maximum strain. 
This is called the Castle Creek fault. : 
Silver fault system —At the time that the folding in the beds was going on, 
certain of the more rigid of the formations slipped upon certain others, 
forming a number of bedding faults. Some of these faults were parallel 
to the bedding, while some of the larger ones were not always parallel, 
but ran locally at a slight angle to it. There are many of these faults, 
but they are not often conspicuous, for the very reason of their parallelism 
with the beddmg. In the Weber and Maroon formations they have not 
been carefully traced, although in the Cowenhoven tunnel several faults 
belonging to this system may be observed, a specially well-marked one 
occurring at the contact of the Weber and Maroon. It is probable that 
these faults are more important in the lower sedimentary beds than in the 
upper, being caused by the slipping of the strata over the underlying, more 
rigid granite. The most important fault of this system—the Silver fault— 
occurs at the contact of the Weber with the underlying formations, and in 
places has a certain amount of obliquity with the bedding planes. A short 
distance below this fault is the Contact fault, which lies between the blue 
limestone and the dolomite of the Leadville formation. This latter fault is 
apparently much slighter, and, so far as observed, is strictly parallel with 
the bedding. These faults of the Silver system probably originated earlier 
