POST-ABAPAHOE MOVEMENT. 33 



evidence already obtained it appeal's that such lakes existed along the easl 

 front of the Rocky Mountains at Canyon City and in Huerfano Park; on 

 their southern slope in the valley of the Animas; on the west along the 

 flanks of the Elk Mountains: and in the principal interior valleys, the 

 North, South, and Middle parks. 



To the northward, along the east trout of the mountains, the continuity 

 of the post-Laramie exposures is broken in northern Colorado and southern 

 Wyoming by a covering of .Miocene and Pliocene beds, but they reappear 

 in the basin of the Cheyenne River, and are abundantly exposed in the 

 valleys of the tributaries to the upper Missouri. 



DENVER FORMATION. 



The beds deposited in the Denver Lake reached a thickness of over 

 1,40<> feet along the Hanks of the mountains, but were probably somewhat 

 thinner toward the middle of the basin. The total thickness of the beds 

 as originally deposited can uo longer be determined, owing to the extensive 

 erosion to which they have since keen subjected. 



The most striking characteristic of these beds is the extent to which 

 debris of greal varieties of andesitic lavas enter into their composition. 

 The lower 400 feet of the series are composed entirely of eruptive debris; 

 above this point Archean and sedimentary debris are found in small but 

 increasing proportion, and above 900 feet the material derived from the 

 abrasion of Archean rocks is largely predominant, eruptive debris being 

 still present in small amount, however, to the highest remaining part of 

 the beds. The distribution of these varying constituents shows that the 

 eruptive material must have come from the mountains to the west of tin- 

 lake. The Archean material contains large bowlders, and the sand grains 

 are angular. With the first appearance of Archean debris are found a lim- 

 ited amount of pebbles, traceable to the Dakota conglomerate and Laramie 

 sandstones. 



That the Denver beds were deposited in shallow waters is shown by 

 the frequent cross bedding observable both in sandstone and conglomerate, 

 and by the plant remains and standing tree stumps that abound at certain 

 horizons. 



MON XXVII 3 



