42 GEOLOGY AND MINING INDUSTRY OF LEADVILLB. 



deposited in quiet waters, like those of a lake rather than of a mountain 

 torrent, for which reason it seems probable that a second Lake Arkansas was 

 formed at the very close of the Glacial period, perhaps by the damming up 

 of the valley by a terminal moraine, which in its turn finally broke its bar- 

 riers and was drained of its waters, leaving- a basin-shaped valley in whose 

 original bottom, as represented by the mesa-like spurs, the lower part of 

 the present stream beds have been cut out. 



stream erosion. The valleys of the minor streams which head in the 

 amphitheaters at the summit of the range were shaped out and took their 

 general direction during the Glacial period. It is evident that their upper 

 portions, at the heads of these amphitheaters, have been but little changed 

 by later erosion, since glacial striae are still found in some cases on their 

 present bottoms. 



The amount of erosion produced by rain and running water increases 

 in direct ratio with the distance from the crest of the range. In the valleys 

 of some of the larger streams running down from its summit this erosion 

 *has cut to a depth of 500 feet below the valley bottom left when the gla- 

 ciers receded, and many minor valleys, like California gulch, which do not 

 head at the actual summit, have been entirely carved out by these agencies 

 since the close of the Glacial period. 



Valleys. The valleys of the minor streams, or gulches as they are gen- 

 erally called, may be divided in the vicinity of Leadville into three classes, 

 according to age and manner of formation. They may be distinguished as 

 (1) glacial valleys, (2) valleys of erosion, and (3) surface valleys. 



The first and oldest, which owe their main outline to the carving of 

 glaciers, have in cro?s section a U-shape and head in glacial amphitheaters, 

 from which they pursue a relatively straight course down the mountain 

 slope. Their original form is more or less modified by subsequent erosion. 

 To this class belong the larger valleys, often forming canons on the east 

 side of the range, and the east fork of the Arkansas and Evans, Iowa, and 

 Empire gulches on the west side. 



The valleys of the second class, which have been cut out of solid rock 

 exclusively by the action of running water, have a V-shaped outline in 

 cross section and a winding course, their direction being dependent on the 



