VALLEY EROSION. 43 



unequal resistance offered by the peculiar position or texture of the rocks out 

 of which they are carved. They also want the amphitheater-shaped head 

 which characterizes the first class. They are more recent than the glacial 

 valleys and have sometimes been cut out of their bottoms. The most 

 striking example of this kind of valley is California gulch. 



The third class, which are of the most recent formation, are likewise 

 valleys of erosion ; but they have been cut, not out of solid rock, but out of 

 recent surface accumulations like the Lake beds, which have not yet be- 

 come solid rock. They are relatively broad and shallow and are often dry 

 for a great part of the year. They are like the shallow ravines and river 

 valleys of the Great Plains and of the Nevada valleys, and like them proba- 

 bly mainly carved by sudden freshets. Little Evans, Georgia, and Thomp- 

 son's gulches are valleys of this class. On the map of Leadville and vicinity 

 it will be seen that the geological outlines cross these valleys without the 

 re-entering angle which they have on the lines of the, other valleys. 



Little Evans Valley drains the amphitheater on the south face of Pros 

 pect Mountain, being separated from Big Evans Valley only by a moraine 

 ridge formed by the glacier of the second epoch. It is thus proved that the 

 amphitheaters were carved out by the earlier set of glaciers, since the glacier 

 from the Prospect Mountain amphitheater was originally a branch of the main 

 glacier from the Evans amphitheater, and it was the moraine of the second 

 Evans glacier which, being placed across the mouth of the Prospect Mount- 

 ain amphitheater, necessitated its seeking a new outlet for its waters. That 

 at one time ice must have filled the amphitheaters to their brim, and been 

 in places over 2,000 feet thick, is proved by their configuration and by the 

 position of erratic blocks. 



In the region shown on the accompanying maps, the two main glaciers 

 of the second epoch were the Evans and the Iowa. The latter had three 

 heads, but its lower portion, as shown by the lateral moraines which remain 

 on the sides of the present gulch, was straight and narrow. The later 

 Evans glacier, however, spread out as it descended, having left a prominent 

 moraine ridge along the north bank of the present stream at the foot of 

 Prospect Mountain, while on the south side a somewhat disconnected moraine 

 ridge follows approximately the course of Stray Horse gulch, the moraine 



