p. B. KING 



The lower intermontane belt begins on the north with the wide 

 depression along the Yukon River in Alaska, continues southward 

 through various plateaus and ranges in Yukon Territory and British 

 Columbia, and includes the Columbia Plateau, Colorado Plateau, 

 and Basin and Range province in the western United States. 



The eastern mountain chain is represented in Alaska by the Brooks 

 Range, and farther south, in Canada and the western United States, 

 by ranges that go under the general title of Rocky Mountains. The 

 Rocky Mountains end as a continuous barrier in northern New 

 Mexico, and for considerable distances southward the eastern ranges 

 are discontinuous and are of Basin and Range type. 



In Mexico, the tripartite division of the Cordillera farther north 

 is lost. Here the Cordillera is essentially a high plateau, breaking off 

 in ranges on the east, west, and south, which form the three Sierra 

 Madres of that country — Oriental, Occidental, and del Sur. 



These are the gross geographic forms of the Cordillera today, yet 

 to some extent they are accidental, and at most reflect only latest 

 warping of the crust. They have slight utility in an analysis of the 

 evolution of the surface features of the Cordillera, as each consists 

 of diverse geological features that have developed at different times 

 and in different ways. For example, the Sierra Nevada is an uplifted 

 block of crystalline rocks, whereas the Cascade Range, its orographic 

 continuation on the north, was built primarily by volcanic erup- 

 tions. 



Present surface forms, rocks, and structures of the Cordillera have 

 developed through a long span of geologic time. Mountains arose 

 first in the western part of the region, in the middle of Mesozoic 

 time; others farther east developed in later Mesozoic and early 

 Tertiary times. Subsequently, in Tertiary and Quaternary times, 

 the initial mountains of the whole region were modified into the 

 forms we now see — by a continuation of crustal mobility, supple- 

 mented by volcanism, erosion, and sedimentation. The eastern part 

 of the Cordillera appears to be attaining stability now, so that the 

 modifying processes are becoming less active. On the west they are 

 still at work, as may be seen by the seismic and volcanic unrest near 

 the Pacific Coast, and one may anticipate continuing rearrangements 

 of the geography there. It is my purpose in the pages that follow to 

 elaborate on the sequence of events thus briefly outlined. 



