EVOLUTION OF MODERN SURFACE FEATURES 25 



formed in forested floodplains and swamps, in a warm humid climate, 

 probably at an altitude no more than a thousand feet above sea level ; 

 Paleocene rocks contain small mammals that were members of an 

 arboreal forest community (Van Houten, 1948, p. 2105). 



The ranges that intervened between the areas of deposition were 

 outlined in much their present form during the Laramide orogeny, 

 but could not have projected to their present bold heights, or they 

 would have prevented an ingress of moisture-laden winds from the 

 west and the climate would have been much drier (Mackin, 1937, 

 p. 819). More likely, their summits rose no more than a few thousand 

 feet above the basin floors ; their rocks were greatly uplifted by the 

 orogeny, but were worn down nearly as rapidly as they arose. 



POST-OROGENIC PHASE 



We pass now to that part of our story which is perhaps of greatest 

 interest to this audience — the shaping of the Cordilleran mountain 

 belt after the orogenic phase, in Tertiary and Quaternary times. 



General Concepts 



Crustal unrest continues in a mountain belt long after the orogenic 

 phase. The orogenic phase itself created many modifications in the 

 crust and subcrust, which were stable so long as the region was in the 

 grip of strong compression. Relatively light sialic crust might be 

 thickened as a root beneath the mountain belt, and sialic crust might 

 be added along the oceanward edge where only simatic crust was 

 present before. Although the orogeny generally thickened and 

 strengthened the continental crust, it also produced transverse flaws 

 and zones of weakness, by differential movement between adjoining 

 segments of the belt. 



With relaxation of compression, there was a shift to a new equi- 

 librium. Overthickened parts of the crust might have risen buoy- 

 antly, or the mountain root might have been dissipated by subcrustal 

 transfer of material. Further movements might have followed the 

 zones of weakness, or such zones provided routes for the ascent of 

 magmas. 



These processes of readjustment are complex, not well understood, 

 and much debated, but their surface manifestations are more evident 

 and can be read in the rocks, their structures, and in the changes in 



