26 P- B. KING 



the landscape. Such modifications of the Cordilleran region have 

 been perhaps as great as in any other mountain belt, and include : 



1. Continued differential movements between mountain up- 

 lifts and intervening basins in the epochs immediately succeeding 

 the orogeny, or perhaps in the waning stages of the orogeny. Some, 

 but not all, of the basins in the central and southern Rocky Moun- 

 tains were thus accentuated during Eocene and Oligocene time. 



2. Breakup of extensive areas of the deformed terrain by block 

 faulting to produce a succession of mountains and intervening 

 basins. Such was the fate of the former miogeosynclinal area and the 

 eastern part of the eugeosynclinal area in mid-section in the United 

 States — in the Great Basin — but similar structures extend far to the 

 south and southeast through the more inclusive Basin and Range 



province. 



3. Regional uplift of broad areas without much folding or fault-" 

 ing, especially late in the post-orogenic period. The greatest of these 

 uplifted regions encompassed virtually all the central and southern 

 Rocky Mountains, the Colorado Plateau on the west, and the Great 

 Plains on the east ; smaller and more complex areas of regional up- 

 lift occur farther west, as in the Sierra Nevada and Cascade Ranges. 



4. Volcanic activity and accompanying shallow intrusions. Some 

 volcanism took place locally in the Cordilleran region even during 

 the orogenic phase; in the post-orogenic phase it occurred in varying 

 degree, at one time or another, in almost every part of the region. 

 Volcanism was concentrated more in some areas than others, how- 

 ever, and in some of them was long persistent. By far the most 

 voluminous and persistent volcanism was in the northwestern 

 United States, in the Columbia Plateau and Cascade Range. 



5. Development of faults along which movements were not up- 

 ward and downward, but along which one side moved laterally 

 against the other. The most famous of these is the San Andreas 

 fault of California, but many more occur near it and others else- 

 where; some, perhaps, are as yet undetected. 



6. Continued orogeny, marked by local subsidence and sedimen- 

 tation, uplift, and deformation along the ocean ward side of the 

 mountain belt. In the Coast Ranges of California, strata as young as 

 the Pliocene and Pleistocene are strongly folded in places, and the 

 seismic record indicates that the crust is still unstable. 



