EVOLUTION OF MODERN SURFACE FEATURES 55 



Cordillera, broadly in mid-I\Iesozoic time — in places in the Jurassic, 

 elsewhere somewhat later. Rocks formed in this part of the geo- 

 syncline were deformed, metamorphosed, and invaded by large 

 bodies of magma. The deformed rocks were raised into a land sur- 

 face, from which detritus was shed westward into the Pacific Ocean 

 basin, and eastward as a broad sheet into the interior of the con- 

 tinent, across the remainder of the geosyncline. 



During Cretaceous time, deformation progressed eastward from 

 the initial disturbed belt, folding and faulting the rocks of the Great 

 Basin area, more lightly affecting those on the site of the Colorado 

 Plateau, and more heavily affecting those in the Rocky Mountains 

 beyond. In the southern part of the Rocky Mountains, zones of 

 weakness had already been created by mountain-making during 

 Paleozoic time. By the close of the orogenic phase, in Late Cre- 

 taceous and Paleocene times, deformation had reached the edge of 

 the present Great Plains, but it progressed no farther inland. 



The folding and faulting of the orogenic phase did not produce the 

 modern topography. While the surface was raised and lowered by it, 

 leveling processes of erosion and sedimentation were active and pre- 

 vented development of strong relief; moreover, regional altitudes 

 remained low. 



Modern surface features evolved by a multitude of crustal proc- 

 esses during the post-orogenic phase, in Tertiary and Quarternary 

 times. Intermontane basins subsided (as in Wyoming and Colorado), 

 large areas were broken up by block faulting (as in the Great Basin), 

 other large areas were overspread by lava (as in the Columbia 

 Plateau), and mountains were formed by the building of chains of 

 volcanoes (as in the Cascade Range). Besides, extensive regions 

 were uplifted relative to their surroundings, with little internal 

 deformation. The largest uplifted region centered in the Rocky 

 Mountains and extended into the Great Plains and Colorado 

 Plateau; it was raised mainly before later Tertiary time, but with 

 diminishing uplifts into the Pleistocene. Smaller, more complex 

 uplifts took place somewhat later in the Sierra Nevada and Cascade 

 Range; in the Sierra Nevada, uplift was accompanied by marked 

 faulting along the eastern side. 



The post-orogenic (Tertiary and Quaternary) movements raised 

 the Cordilleran region to its present generally high altitude. Streams, 

 quickened by the uplift and by increased rainfall during the Pleisto- 



