PLEISTOCENE SUBMERGENCE IN COLUMBIA VALLEY 503 



from the distributary channels across it of a lowering water body 

 during its formation all suggest that it was deposited after the 

 maximum of the Pleistocene submergence. This may have been 

 either during a pause in the subsidence of the water, or in a later 

 and lesser submergence. The first interpretation is preferred. 



THE SUBMERGENCE ELSEWHERE IN WESTERN WASHINGTON 



So great a submergence in late Pleistocene time could not have 

 been limited to the lower Columbia drainage. Its duration was so 

 brief, however, that it is not definitely recorded by shore features 

 or lacustrine plains, and had there been no bergs on it the occur- 

 rence perhaps and the extent certainly never would have been 

 known. 



There appears scant hope, therefore, of finding records of the 

 submergence elsewhere. The only other region in the Pacific 

 Northwest where a similar episode has been reported is the lowland 

 of Puget Sound. During the latest, or Vashon, glaciation of Puget 

 Sound the sea-level was not more than 75 feet above its present 

 altitude.^ But after retreat of the ice from the lowland, marine 

 waters submerged the northern part of the region to a depth of at 

 least 290 feet A.T.^ This is recorded by the presence of marine 

 shells in stratified clay overlying the Vashon till sheet. 



Erratic bowlders cannot be used for this study in the Puget 

 Sound Valley or its margining slopes because glacial ice has covered 

 the whole region. South of the terminal moraine of the Puget 

 Sound Glacier, however, they would be as valuable a criterion as 

 in the Columbia Valley. But, so far as known, there are no such 

 bowlders in the unglaciated lowlands of southwestern Washington. 

 This may be charged to lack of intensive search for them, or to 

 absence of bergs in such waters. 



GENERAL CONSIDERATIONS 



The highest known erratic bowlders in Willamette Valley 

 (323 feet A.T.) and the highest known marine shells in Puget Sound 

 (290 feet A.T.) mark the known upper limit of the submergence 



^ J H. Bretz, "Glaciation of the Puget Sound Region," Wash. Geol. Surv. Bull. 8 

 (i9i3),p. 229. 



''Ibid., p. 233. 



