48 p. B. KING 



with greater rainfall than the country farther south, and its volume 

 far exceeds that of any other stream on the western slope of the 

 Cordillera. Similar greater rainfall probably prevailed in this north- 

 ern segment of the Cordillera through much of the post-orogenic 

 phase, so one may assume that an ancestor of the Columbia existed 

 there throughout much of the Tertiary and Quaternary. 



With onset of the eruptions of Columbia River basalt, the course 

 of the river in the coastal plain downstream from the highlands of 

 the Nevadan orogenic belt was obliterated by the flood of lava. Parts 

 of the river and its tributaries in the highlands to the north and 

 northeast were dammed by the lava, and a series of lakes were 

 formed. Each lake drained around the spur ends of the highlands of 

 older rocks to the next lower lake on the west, thus establishing an 

 exit for the waters along the edge of the volcanic field ; with down- 

 cutting, there was thus established the new course of the Columbia, 

 part of which it still follows (Fig. 9). Farther downstream in south- 

 ern Washington, however, the river is deflected eastward into the 

 lava country, probably as a result of outbuilding of andesitic debris 

 from the Cascade Range on the west (Waters, 1955b, pp. 681-683). 



In southern Washington the river also crosses several anticlinal 

 folds in the basalt, which plunge southeast from the Cascade Range. 

 Each anticlinal fold is expressed topographically as a ridge, and in 

 each ridge the river and several of its tributaries have cut deep 

 gorges. The Columbia and its tributaries were probably antecedent 

 to the anticlines (Waters, 1955b, pp. 679-681) — they had much their 

 present courses before the folding and maintained them by down- 

 cutting as the anticlines were raised. Still farther downstream the 

 river cuts a much larger gorge through the complexly upbuilt and 

 upwarped Cascade Range. Many geologists believe that the river is 

 antecedent to the growth of the Cascade Range also, but some 

 would ascribe its course through the mountains to a complex process 

 of superposition (Hodge, 1938, pp. 888-918). 



Pacific Border of Cordillera (Coast Ranges of California) 



General Geography and Geology. From the Klamath Mountains 

 southward through California and into Baja California, the western- 

 most ranges of the Cordillera are the Coast Ranges along the Pa- 

 cific Ocean (Fig. 10). Southward to Point Conception they consist of 

 numerous parallel ridges, each diverging south-southeastward from 



