GLACIAL EROSION. 41 



inferior importance to the great parks themselves, which date back to pre- 

 Cambrian time. 



Glacial erosion. The detrital materials brought down from the adjoining 

 mountains and deposited along the Arkansas Valley during the Glacial 

 period show that the general form of the latter had already been determined 

 before that time. These deposits, though of similar origin and lithologcal 

 character, belong to two distinctly marked epochs. Those of the former 

 constitute the so-called Lake beds, formed of detrital and mainly morainal 

 material, brought down from the mountains by the freshets which occurred 

 during the melting of the ice at the close of the first cold epoch of the 

 Glacial period, and which formed stratified deposits at the bottom of the 

 great lake at the head of the Arkansas Valley, which will be called the 

 Arkansas Lake. These beds, which reached a thickness of at least three 

 hundred feet, are now found on either side of the alluvial bottom of the 

 present stream, forming the base of the mesa-like terminations of the mount- 

 ain slopes and in some cases extending to an elevation of 1,000 feet above 

 the present valley bottom, a height to which the angle of the deposition of* 

 the beds could hardly have carried them and which gives evidence that the 

 elevation of the range has continued in a modified degree since Glacial 

 times. After the draining of this lake, in some manner not now to be 

 traced, a second epoch of glacier formation set in, during which the new 

 glaciers occupied the same positions as the older ones and continued the 

 work of grinding and valley carving. They extended out over the Lake 

 beds deposited during the warmer period, as proved by the present position 

 of the lateral moraines of Iowa and Evans gulches. An immense amount 

 of detrital material must have been accumulated on the slopes of the range 

 by this second system of glaciers, and during the floods and freshets that 

 must have accompanied their melting and recession this material was par- 

 tially rearranged and spread out over the lower part of the Leadville region, 

 both above the already existing Lake beds and in some cases over rock sur- 

 faces not previously covered by these deposits. This rearranged moraine 

 material has received the local name of "Wash." From the present regular 

 and even surface of the lower spurs, where the Wash lies conformably over 

 the Lake beds, it is evident that the former, like the latter, must have been 



