THE BUILDING OF THE COLORADO ROCKIES 147 



Appalachian mass in central Pennsylvania was that of a triangular 

 prism, or wedge, which apexed downward in the middle of the range. 

 Though the wedge-shaped mass was a result quite unexpected, 

 it did harmonize well with various theoretical considerations. In 

 fact it not only appeared reasonable of itself, but seemed to suggest 

 that the principles developed might well be of very general appli- 

 cation to mountain building, and might also find some application 

 to plateaus and continental sectors. The wedge-shaped block 

 and wedge dynamics in deformation seemed to open up possi- 

 bilities along a new line. In view of this, it seemed desirable to 

 apply this method of inquiry to some other mountain systems. 

 For this work the Rocky Mountains were the most available, 

 and as a result of preliminary scouting in the summer of 19 13 the 

 section in the vicinity of the Denver and Salt Lake Railroad 

 (Moffat Road) in northern Colorado was selected for the trial. 

 The strip from the Great Plains near Lyons to the Grand Hogback 

 at Glenwood Springs affords perhaps the most characteristic section 

 which can be selected, and is at the same time readily accessible. 

 The special field work for this cross-section was done largely in the 

 summers of 19 15 and 1916, besides which short supplementary 

 visits were made in 19 17 and 19 18, including many trips to points 

 off the section for collateral suggestions. 



THE COLORADO ROCKIES 



The Rocky Mountain system extends from the Endicott Range 

 of northern Alaska southward as far as central New Mexico, 

 where it gradually fades out. It is a belt of varying width and 

 structure. In Alberta and British Columbia the Rocky Mountain 

 system, according to Daly, is very definitely bounded by the Great 

 Plains on the east and the Rocky Mountain trench on the west.^ 

 It is bordered on its eastern margin throughout many degrees of 

 latitude by great overthrust faults. In its western part, though 

 faults continue to be numerous, folding becomes more prominent.' 

 Characterized thus structurally, the Rocky system continues 



' R. A. Daly, "Geology of the North American Cordillera at the Forty-ninth 

 Parallel," Geol. Siirv. Can., Memoir 38 (1912), PI. Ill and p. 27. 



^ R. G. McConnell, "Report on the Geological Features of a Portion of the Rocky 

 Mountains," Geol. Siirv., Can., II (1886), Part D, pp. 31 to 40. 



