86 



The exact date at which elevation of the Rocky Moun- 

 tains commenced is not certain, but it is probable that from 

 early Cretaceous times the crust here was under strain 

 and that at intervals during the warping of the crust 

 before the close of that period the western part of the range 

 had been marked out by hills which were being denuded of 

 their top covering of shales, quartzites, and limestones 

 to swell the accumulations in the Cretaceous sea to the 

 east. The period of mountain building to which the 

 elevation of the Rocky Mountains is assigned, the Laramide 

 revolution, is probably a long one. The formation 

 of the outer ranges with their frequent great overthrusts 

 eastward, was subsequent to the elevation of the ranges 

 of the watershed or to the deposition of the early Tertiary 

 beds of the Paskapoo formation. 



The denudation of post-Tertiary times has removed 

 most of the broken material resulting from this late revol- 

 ution, but the large well rounded pebbles in the Oligocene 

 beds of the Cypress and Hand Hills, are probably the 

 remains of that material. This period of mountain 

 building is probably later than the Laramie and occurred 

 between the Paskapoo or early Tertiary and the Oligocene. 



Part of the denudation of the early Tertiary and 

 Cretaceous beds of this basin may have been accomplished 

 at this time, especially in the part near the mountains, 

 but the greater part was due to a general elevation in 

 Pliocene times, when much of the area was in process of 

 reduction. The amount of material removed is well 

 shown on the north side of Cypress Hills, where from the 

 level of the Oligocene deposits the Saskatchewan river 

 is now cutting through horizontal beds that are 2,000 

 feet (608 m.) below. The wasting away of material from 

 the edge of the basin was also continued and before Glacial 

 time the plateau of Cretaceous deposits assumed nearly 

 its present form. 



Many of the present valleys are broad depressions 

 formed in pre-Glacial time, and sometimes show old stream 

 gravels, derived from the Oligocene conglomerates, covered 

 by the boulder till. Glacial deposits are spread over all 

 the area, and, almost to the mountains, hold erratics 

 derived from a northeastern source. The Cordilleran 

 glacial material has been carried but a short distance 

 eastward from the mountains. 



