478 



STRUCTURAL GEOLOGY OF NORTH AMERICA 



Fig. 29.19. Tectonic map of the late Pliocene and Quaternary crustal movements along 

 the Pacific. 



sagging, and warping type, and therefore contrasts sharply with the close 

 folding, thrusting, and wrench faulting in the California Coast Ranges. 

 The main orogeny of the southern ranges as previously pointed out 

 occurred in late Pliocene and mid-Pleistocene time, but it mostly escaped 

 the Washington and Oregon ranges. On the other hand, the late Miocene 

 deformation seems to have been about of equal intensity both north and 

 south of the Klamaths. 



The gentle archings have been deduced from several lines of evidence. 

 The first and most conspicuous is the parallelism of the three major 

 topographic features, namely, the Cascade Range, the Willamette-Puget 

 Sound depression, and the Coast Range. The two ranges are taken as 

 arches or broad, gentle anticlines, and the depression as an intervening 

 broad, gentle syncline. The second line of evidence comes from erosion 

 surfaces, both inland and coastal. The third concerns the glacial deposits, 

 which are very extensive in parts of the Puget Sound depression, and 

 vertical crustal movements associated with the glaciation. 



According to Weaver (1931-37) the Pliocene deposits, where known, 

 rest unconformably upon the Miocene and are much less tilted. Thus 

 the late Miocene phase is dated. During the latest Miocene and Pliocene, 

 minor differential movements allowed the oceanic waters to transgress 

 easterly and cover small restricted areas on the western side of the 

 Olympic peninsula and in the coastal portion of southwestern Oregon. 

 All other areas were undergoing erosion, and it is probable that the 

 major channels of Puget Sound, such as Hood Canal, Admiralty Inlet, 

 Georgia Strait, and the Strait of Juan de Fuca, were being excavated. The 

 marine waters that occupy these valleys at the present time gained access 

 as the result of Pleistocene depression just preceding and during the 

 glacial epoch. 



Near the close of Pliocene time the two broad anticlines and inter- 

 vening syncline developed and emphasized the individuality of the Coast 

 and Cascade ranges and the Puget trough (Weaver, 1937). See map, 

 Fig. 29.19. These north-south structures were probably superposed on 

 the Miocene northwest trending folds. The Cascade Mountains ultimately 

 attained their present elevation during the early Pleistocene, and upon 

 their surface was built a row of majestic volcanic cones such as Mount 



