468 



ISLAND RANGES 



STRUCTURAL GEOLOGY OF NORTH AMERICA 

 INTERMONT VALLEY BELT MAINLAND OR CA5CADE RANGE 



5,000' 



BRITISH COLUMBIA AND SOUTHEASTERN ALASKA 







UPLAND SURFACE 



KLAMATH MOUNTAINS AND NORTHERN CALIFORNIA 



Fig. 29.12. Idealized diagrams to represent vertical movements of the crust in Pleistocene time 

 along the Pacific coast. The upper diagram is schematic for the coastland of British Columbia 

 and southeastern Alaska. It runs east-west, and the U-shaped valley is representative of the 

 many great fiords that trench the upland. The lower diagram is schematic for the Klamaths of 

 northern California and for the coastland of this area. It should be considered as a north- 

 south section in the Klamaths with the horizontal lines representing sea level at different times 

 along the coast. The horizontal lines in both diagrams represent different sea levels. Sea level 1 

 was the base to which the high surface in both regions was graded. Sea level 2 was the one after 



land and dissected by streams flowing over it. In the lower diagram, Fig. 

 29.12, the horizontal datum line marked 2 indicates sea level at this time. 

 The uplift was probably over 2000 feet in the Klamath area. Then fol- 

 lowed a subsidence of over 1500 feet. Datum line 3 indicates the sea level 

 at this stage. The oldest beaches known in the region were established at 

 this time. The highest are 1500 feet above the present sea level. They 

 remain only in remnants today. The deep and narrow valleys cut in stage 

 2 were partly alluviated in stage 3. Through a succession of uplifts, 

 beaches were formed at successive levels down to the present, with the 



the great emergence to which the deep gorges were eroded. Sea level 3 was the one after the 

 great submergence to which the highest beaches now remaining were eroded. Sea level 4 is the 

 present one after appreciable emergence. In British Columbia and southeastern Alaska this last 

 emergence has only recovered 600 feet of the previous 1600 feet of submergence, whereas in 

 nothern California the recovery has been almost complete. The original great uplift was caused 

 undoubtedly by deep-seated crustal disturbances, but the later submergence and emergence were 

 due to isostatic adjustments to the loading and unloading of the glaciers. 



modern coastal plain not far above sea level as the last major beach. 



Northward from the Klamaths in southern Oregon, the shore terraces 

 gradually disappear. The same is true southward in northern California. 

 The most recent submergence north of the 40th parallel can be detected 

 in the tidal portions of the rivers which are somewhat drowned. The 

 subsidence increases as far north as the Columbia. 



These very considerable epeirogenic movements in late Pleistocene and 

 Recent time must be viewed with respect, when the offshore submarine 

 topography is considered, because they show how possible it is for ex- 



