THE BUILDING OF THE COLORADO ROCKIES 235 



Colorado Rockies are therefore mountains in which the horizontally 

 operating forces have achieved only moderate results. Farther 

 north in Montana and Alberta, however, the great overthrust 

 faults on the Rocky Mountain front imply that the horizontal 

 thrusting has there been much greater. 



THE ELEVATION OF THE FOLDED TRACT 



When a region is folded by compressive stress, the horizontal 

 .shortening is compensated by vertical bulging, except in so far as 

 there be an actual reduction in the volume of the material deformed. 

 According to our fundamental postulate, if the amount of crustal 

 shortening and the amount of the resulting vertical bulge be known, 

 the thickness of the folded shell may be calculated. It is next in 

 order to determine, if possible, the amount of vertical uplift which 

 has resulted from the folding. 



The first requisite for the determination of the amount of 

 upswelling is a datum-plane above which the heights of the folds are 

 to be measured. This datum-plane should coincide in position with 

 the original surface of the region just before the folding commenced. 

 The chief difficulty is to locate a datum-plane available today which 

 will correspond in position to the original surface before folding. 

 Mountains ideally built for such a study should be folded from a 

 plain which was just at sea-level. A minimum of epeirogenic 

 movement should accompany the orogenic disturbance. Then 

 without the entrance of any other diastrophic changes to add com- 

 plications, erosion should again reduce the folded tract completely 

 to base level. This base level should correspond as closely as 

 possible to the sea-level from which the mountains originally 

 rose. This base level would then constitute the proper datum- 

 plane above which to measure the heights of the folds. 



The history of the Colorado region has been such as to render 

 it rather well suited to a treatment of this sort. The closing stages 

 of the Cretaceous period in Colorado witnessed a transition from 

 the conditions of marine sedimentation to those of terrestrial 

 deposition. Among the last beds laid down before the inauguration 

 of the Laramide folding were vegetable accumulations which 

 have since become coal. They were presumably formed in marshes 



