34 p. B. KING 



Eocene and middle Tertiary times the eastern Cordillera was proba- 

 bly arched upward as a unit by as much as 5,000 feet. During the 

 same period, the climate became more arid, partly because of a 

 world-wide secular change (Axel rod, 1957, pp. 40-41), partly because 

 the crest of the uplift created a rain shadow over the area to the east. 



Climax of uplift and aridity was probably also the climax of 

 aggradation in the areas between and east of the mountains. It ap- 

 pears well established (Atwood and Atwood, 1938, pp. 965-968; 

 Mackin, 1937, pp. 821-823) that extensive areas not now covered by 

 later Tertiary deposits, including the earlier Tertiary intermontane 

 basins and the lower mountain ends and spurs, were then buried. 

 Most of the emergent areas were planed to form the Rocky Moun- 

 tain peneplain, leaving, as projections above the general level, only 

 the unreduced peaks along the axes of the ranges. 



Opinions differ as to the relief of the aggraded and planed-off 

 surface of later Tertiary time. The view of many geologists has been 

 that regional relief of the eastern Cordillera was considerably less 

 than that of today, and that modern regional and local relief is the 

 product of renewed uplift during the Pleistocene. Other geologists 

 call attention to the fact that preserved gradients of the Great 

 Plains deposits and of the Rocky Mountain peneplain are about 

 those to which stream regimen would have been adjusted in an arid 

 climate and, by extrapolation, infer that the late Tertiary graded 

 surface had almost the same regional relief as that of the present 

 country. Regional relief had thus increased greatly from that of 

 early Tertiary time, but local relief was about as subdued, and 

 differed much from the present strong local relief. 



Quaternary Denudation and Dissection. Transformation of the 

 late Tertiary landscape into that of the present was thus an event 

 of the Quaternary period, mainly of the Pleistocene epoch. Regard- 

 less of ultimate causes, it resulted from accelerated stream erosion, 

 which degraded and dissected the whole region. 



Such erosion removed large volumes of upper Tertiary deposits 

 from the mountain areas, and excavated the rocks beneath by vary- 

 ing amounts according to their resistance. The basins, formed of 

 weak Cretaceous and lower Tertiary rocks, were etched out, so that 

 the ranges of pre-Cambrian crystalline rocks and Paleozoic stratified 

 rocks projected above them. During the period of aggradation 

 streams had wandered at will down the slopes of the subdued sur- 



