THE BUILDING OF THE COLORADO ROCKIES i6i 



river valleys which have cut well below this level; east of Long's 

 Peak the plains have an elevation of only S,ooo feet above the 

 sea. The greater elevation of the Front Range in Colorado is 

 the result of greater upbowing of the old peneplain surface in that 

 latitude. 



On the Sherman (Wyo.) sheet close to the Wyoming-Colorado 

 line, the Sherman peneplain level is in the neighborhood of 7,700 

 feet above the sea. On the Livermore (Colo.) sheet next south 

 the plain rises from about 7,000 near the sedimentary hogbacks to 

 8,000 feet at the west margin of the sheet. On the Mt. Olympus 

 sheet next south the general elevation of the plain is not very 

 different, but on the Boulder sheet adjoining Mt. Olympus on the 

 south the plain rises from 6,500 feet elevation near the city of 

 Boulder to approximately 9,000 feet at the west margin of the sheet 

 near the mining camp of Ward. While there may well be some 

 minor levels included, these elevations constitute a very evident 

 erosion plain which slopes toward the east. The slant of the 

 plain becomes progressively steeper as it is followed southward from 

 Sherman, Wyoming, to Boulder, Colorado, indicating a greater 

 up warp of the range in the latitude of Boulder. 



Da\ds considered this old erosion plain to have been once 

 continuous with the so-called summit peneplain whose remnants 

 are today seen on the continental divide at an elevation of 12,000 

 feet.^ If so, it would mean that the incHnation of the plain becomes 

 greater as it nears the divide. It is natural enough that there 

 should have been sharper tilting along the axis of the range. In 

 the region crossed by my section, however, the evidence did not 

 seem to be altogether conclusive. There may be two plains 

 representing different cycles. If so, this would suggest some 

 upwarping between and would have a bearing upon the age of the 

 summit peneplain. But if, on the other hand, the summit plain 

 and the lower plain to the east are truly parts of the same surface 

 of denudation, the rather obvious recency of the latter would indi- 

 cate that the summit peneplain is of late Tertiary age. The erosion 

 plain extending from Allen's Park and Ward to the foothills near 

 Boulder must clearly be as late as the Pliocene. 



^ W. M. Davis, op. clt., p. 31 and Fig. B2, pp. 76-77. 



