14 P. B. KING 



along the southeastern side of North America. The zone of weakness 

 that they created in the crust greatly influenced the configuration 

 of the Rocky Mountain structures that developed there later (Bur- 

 bank and Lovering, 1933, pp. 277-301). 



By the first half of Mesozoic time, considerable rearrangements 

 had taken place in the miogeosynclinal area. The land barrier be- 

 tween the miogeosyncline and eugeosyncline, whose development 

 had begun in later Paleozoic time, restricted the extent of Triassic 

 and Jurassic sedimentation on the west; troughs along the eastern 

 side of the barrier received a considerable thickness of marine sedi- 

 ments (Fig. 3). To the east, deposits spread beyond the Paleozoic 

 miogeosyncline, wedging out in the Great Plains area east of the site 

 of the Rocky Mountains. At least in the south, most of these eastern 

 deposits were land-laid, and in the Colorado Plateau include such 

 characteristic units as red stream deposits of the Triassic (Moenkopi 

 and Chinle formations), great sand dune deposits of the Late Triassic 

 and Early Jurassic (Wingate and Navajo sandstones), and vari- 

 colored stream deposits of the Late Jurassic (Morrison formation) 

 (Baker et al., 1936, pp. 48-55). 



During Cretaceous time, sedimentation continued in much the 

 same region as that covered by the Triassic and Jurassic deposits, 

 but the Cretaceous deposits are so closely related to the orogenic 

 phase of Cordilleran history that it is best to discuss them later. 



OROGENIC PHASE 



General Concepts 



Terminology. The word orogeny means mountain building, yet to 

 a degree its use has been perverted by geologists. During early work 

 in mountain regions, geologists observed everywhere the strong 

 disturbance of their rocks and assumed that such disturbances were 

 the cause of the mountains themselves. Now we know that these 

 disturbances, while an essential step in the process, did not produce 

 strong mountainous relief; such relief was only acquired later, in the 

 post-orogenic phase. Use of the term orogeny for the deformative 

 phase of mountain growth has nevertheless persisted for want of a 

 better name; it is convenient to use it here. 



The Orogenic Process. The geosynclinal phase of mountain 

 growth blended, as stated, wath the succeeding orogenic phase, when 



