66 GEOLOGY OF ASPEN MINING DISTRICT, COLORADO. 
east. A short distance below this point of overturning the beds resume their 
normal succession very suddenly, and the steep easterly dip changes to a 
comparatively flat westerly one. The beds having this flatter dip form 
the eastern part of the bottom of the syneline. 
At the point where the steep dip of the overturned beds changes to the 
normal westerly dip there is a slight fault, as if the change in position had 
been too abrupt for flexure without breaking. This breaking is not marked 
by any single plane, but by several parallel ones; the chief plane of move- 
ment has been called the Aspen fault. On the east side of this fault the 
beds are vertical or have a very steep dip, which may be either normal or 
reversed, while to the west of the fault the beds have a comparatively flat 
dip, and appear in east-west sections as nearly horizontal. The dip of the 
fault, as shown in the sections, is, as a rule, nearly vertical, becoming in 
depth often a little easterly, and on approaching the surface flattening out 
and acquiring a dip to the west, which upward grows progressively less. 
Where, therefore, the Aspen fault is encountered in the lower workings of 
the mines, it is, with reference to the beds lying east of it, which are vertical 
or nearly so, somewhat in the nature of a bedding fault, while it cuts across 
the flat-lyimg beds on its west side at nearly right angles. Higher up m 
these mines and near the surface, however, the beds on opposite sides of the 
fault become more nearly alike in their attitude, both acquirmg a decided 
but generally not an extreme westerly dip. In passing upward into these 
rocks the Aspen fault seems also to change and to approach more and more 
nearly coincidence with the bedding. In this way much of its throw, which, 
even in the deepest parts of the mine, is not found to be very great, is lost, 
the movement being taken up along the bedding planes of the strata. It 
therefore becomes progressively more difficult to trace, and it very likely 
passes into the Silver fault, as is represented in the 300-foot sections on the 
Aspen Mountain map. The actual nature of this curving fault is hard to 
summarize; it has in general a slight downthrow to the east, which is 
very small, but increases with the difference in dip of the beds on its two 
sides. The greatest actual throw, as measured between two stratigraphical 
planes on opposite sides of the fault, is probably not more than 150 feet, 
and the amount of this throw decreases toward the surface, and also in 
depth, for in depth the rocks again tend to assume more uniform dips on 
both sides of the fault. 
