EVOLUTION OF MODERN SURFACE FEATURES 7 



CONTROLLING PRINCIPLES 

 Nature of Mountain-Building Processes 



Ultimate cause of mountain building is to be sought, not in such 

 merely superficial processes as erosion, sedimentation, glaciation, or 

 volcanism (however much these may shape the landscape in detail), 

 but in forces within or beneath the crust of the earth, which have 

 deformed the rocks and raised or lowered large areas of the surface. 

 Little is known about these forces themselves, but much has been 

 learned about their effects. 



Some of the orogenic phases have been referred to as "revolu- 

 tions," because they are supposed to have brought about drastic 

 rearrangements of the geography and climate, and so modified the 

 environments as to cause far-reaching changes in distribution and 

 kinds of life. Detailed study shows, however, that the different 

 phases merge into each other, and that the changes they brought 

 about were evolutionary rather than revolutionary. Operation of 

 crustal forces was persistent through time, and although there were 

 certain crescendos, development was orderly and progressive, rather 

 than catastrophic. 



Nature of Continental and Oceanic Crusts 



In North America, at least, mountain building was a feature of the 

 edge of the continent — the border zone between the continental 

 platform and the adjacent ocean basins. 



Sequentially, the processes may be divided into an initial or 

 geosynclinal phase, followed by an orogenic phase and a post- 

 orogenic phase, the nature of which will be examined later. The 

 phases were prolonged. In the Cordilleran region the geosynclinal 

 phase endured for at least 350 million years, from Cambrian to 

 Triassic; the orogenic phase, for about 100 million years, from 

 Jurassic to Paleocene; and the post-orogenic phase, for about 50 

 million years, from Eocene to present (Table I). 



Continental platforms and ocean basins are fundamentally 

 different elements of the crust of the earth (the crust is the relatively 

 thin skin of rocks that overlies the dense material of the interior of 

 the earth). Their surfaces stand today at different levels: the con- 

 tinental averages about half a mile above sea level; the oceanic, 3 

 miles or more below sea level. The two levels reflect contrasting 



