PLEISTOCENE GEOLOGY. 257 



westward through a gap in the Archean foothills to the upper and glaciated 

 portion of the Clear Creek Valley, which might have been occupied by 

 such a conglomerate that had since been entirely removed by erosion. 1 

 Whether such a conglomerate existed in this region or not, it is fair to 

 assume from the relic- of' ancient peneplains now existing, such as the mesa 

 region on the Arkansas divide, Raspbeny and Dawsons buttes, and Green 

 Mountain, that, at the commencement of the earlier erosion epoch, the 

 Denver Basin was a gently sloping plain reaching an elevation of about 

 7,500 feet at the present foothills. Whether the erosion commenced before 

 Glacial time or not, it is probable that the greater part of its work was 

 accomplished during that time, when erosive action must have been tar more 

 vigorous and destructive than before or since. 



Modem erosion, as will be shown later, has accomplished but little 

 more than the partial removal of the material that was deposited over the 

 plains area in the intermediate period. An idea of the amount of this earlier 

 erosion may be formed when we consider that the beds of the tortuous 

 V-shaped canyons of the principal streams that to-day issue from the moun- 

 tains, and which have been carved out of the hard crystalline rocks, are 

 for many miles above their mouth a thousand feet lower than the actual 

 summit of Green Mountain, and that the present site of the city of Denver 

 over which the comparatively undisturbed Denver beds then stretched, is 

 now 1,800 feet below the higher members of that formation on Green 

 Mountain. It is impossible now to determine how much thinner these beds 

 became a- the distance from the source of their material along the foothills 

 increased, or to calculate what allowance should be made for the original 

 slope of the beds, hut it is safe to assume that 1,000 to 1,200 feet of these 

 recent beds have been removed from the lower portions of the modern 

 valleys. 



This period must have been one of enormous precipitation as compared 

 with that of the present day, ami its rivers were consequently many times 



'Mr. Cannon is inclined to consider the bowlder beds that at present cap Green Mountain to be 

 relics of such a conglomerate. Mr. Cross, on the other hand, who has made a more detailed stu.h of 

 that mountain, considers them part of the Denvei beds, whose disintegration would amply account 

 for the numerous bowlders of various rocks, mostly Archean, that lie upou its present summit. 

 MON XXVII 17 



