6o8 JOHN LYON RICH 



The rock floor of this plateau is clearly a peneplained surface of 

 very considerable extent and great regularity. We are not dealing 

 in this case with an imaginary peneplain whose existence is deduced 

 from the finding of hilltops at about the same level, but with one 

 in which the original planed surface is still preserved over a broad 

 area with its edges exposed and with an evenness the more striking 

 on account of the folded condition of the underlying rocks. 



Extent of the peneplain. — The portion of the area mapped in 

 detail in the summer of 1908 in which remnants of the peneplain 

 occur measures about twenty-seven miles from north to south and 

 twenty-two miles from east to west. Including Bishop Mountain 

 and the western part of Little Mountain which were not mapped 

 in detail, but which are without doubt parts of the same peneplain, 

 the dimensions just given will be increased by about ten miles in 

 either direction. This makes an area of at least 1,200 square miles 

 which we can definitely say was reduced to the condition of a 

 peneplain of very slight relief. 



There is good evidence that the peneplain had a much greater 

 extent than that indicated by the area over which portions of the 

 gravel-capped plateau still remain; for the lava-sheets of the 

 Leucite Hills, which are scattered over a considerable area from 

 fifteen to forty miles north of Aspen Mountain, lie on the beveled 

 surfaces of the underlying sedimentary rocks. The lava-flows 

 have preserved these beveled surfaces at their original elevations. 

 These, in the case of the larger flows, are so nearly in accord, and 

 so nearly agree with those of the gravel- capped plateaus to the 

 south of Aspen Mountain, that it is reasonably safe to assume that 

 the lavas were poured out on the surface of the same peneplain. 

 The following figures will illustrate this relationship : Approximate 

 elevation of the base of the gravels south of Aspen Mountain, 

 7,600 feet; base of Pilot Butte lava-flow, 7,900 feet; Zirkel Mesa, 

 7,600 feet; Steamboat Mountain, forty miles north of Aspen, 

 8,250 feet; and North Table Mountain, about 8,150 feet. All of 

 these agree in that they, like the plateau to the south, stand from 

 800 to 1,200 feet above the level of the adjacent valleys. Several 

 of the smaller lava-flows are at lower levels and do not agree closely 

 with those mentioned above. A small flow three miles northwest 



