i62 ROLLIN T. CHAMBERLIN 



West of the continental divide along the line of the Grand 

 River section, remnants of a former surface of Httle reHef are 

 observed at various points. In Pliocene times this region, if not 

 brought to the condition of a true peneplain, was at least greatly 

 reduced and many extensive flattish areas developed- Preserved 

 portions of these are seen today on the western uplands of the 

 section at elevations for the most part between 9,000 and 10,000 

 feet. The Lyons-Grand River cross-section given later in this 

 paper seems, however, to belie this statement, but that is because 

 the drawn section closely follows the course of the Grand River 

 and only in a few places does it rise to the upland peneplain 

 level. 



Upbowing. — Since the formation of this late Tertiary peneplain, 

 it has been bowed upward along an axis which corresponds approxi- 

 mately with the present crest of the Front Range. General 

 uplift involving some Httle warp also affected the region west of 

 the Front Range and accentuated the height of several of the ranges. 

 The movement in the Front Range was a gentle tilting in the lati- 

 tude of Long's Peak, though in the foothills it may have been 

 accompanied by normal faulting. Farther south, in the vicinity 

 of Castle Rock and Colorado Springs, Richardson's sections of 

 the Front Range foothills show the Dawson arkose of the early 

 Eocene to be steeply inclined, and cut by a set of normal faults, 

 which are there an important part of the foothill structure.' Finlay 

 has placed this period of disturbance within the PHocene Period.^ 



This upHft has given the Front Range much of its present 

 elevation above the Great Plains to the east. Streams rejuvenated 

 by the uplift have since carved deep canyons, which head far back 

 in the high level plain. Leading back from the plains to the open 

 uplands of Estes Park and Allen's Park, where roads are possible 

 in almost any direction, are the wonderful scenic canyons of the 

 Thompson River and St. Vrain Creek, which are young in the 

 present cycle of erosion. On the west side of the continental 



' G. B. Richardson, U.S. Geol. Survey, Geol. Atlas, Castle Rock Folio, No. 198 

 (1915), pp. lo-ii. 



= George I. Finlay, U.S. Geol. Survey, Geol. Atlas, Colorado Springs Folio, No. 203 

 (1916), pp. 13-14. 



