COAST RANGES OF THE PACIFIC AND THE SAN ANDREAS FAULT SYSTEM 



479 



Baker, Mount Rainier, Mount Adams, Mount St. Helens, Mount Hood, 

 and numerous smaller cones in southern Oregon. See Chapter 33. 



It seems probable that the erosion surface, developed after the late 

 Miocene folding, was itself gently folded, as were the rocks beneath in 

 the late Pliocene archings, and that it was intensely dissected where 

 uplifted most. 



After the elevation of the erosion surface, and after its deep dissection 

 by the voluminous streams of the region, the ice age came on, and is 

 recognized in two stages. During the later advance all the valleys of both 

 the eastern and western slopes of the Cascade Range were filled with ice, 

 which moved downward to lower elevations and built terminal moraines. 

 The valley glaciers in northern Washington entered the Puget Sound 



basin and coalesced with one another, and with the extensive piedmont 

 glacier that had moved southerly between Vancouver Island and the 

 mainland. This great ice floe broke into two tongues; one extended west- 

 erly through the trough of the Strait of Juan de Fuca, and the other 

 moved southward into the southern part of the Puget Sound basin, where 

 it built up a terminal moraine from the southeast corner of the Olympic 

 Mountains easterly to the Cascades. 



After the withdrawal of the ice, the crust has risen in the Puget Sound 

 basin and along the coast of Washington and Oregon from 20 to over 200 

 feet. A most recent submergence has already been noted near the mouth 

 of the Columbia River, and the tidal influence extends eastward to the 

 Cascades. 



