SALIENT FEATURES OF THE GEOLOGY OF OREGON 113 



formation of the block mountains of eastern Oregon. Erosion 

 soon cut these soft Empire sediments nearly to a base level, pro- 

 ducing a second peneplain passing eastward over the Coast Range. 

 Differential uplift within the Klamath and Coast Range regions 

 again rejuvenated the streams, which intrenched themselves, 

 maintaining in some instances their old, meandering courses. In 

 late Pliocene or early Pleistocene these same streams cut deep 

 valleys in the then exposed continental shelf, as is now revealed by 

 soundings. The ancient Willamette River may then have carved 

 in part its wide valley, since deeply filled by alluvium. In the 

 meantime the Cascades had been reduced to a region of low 

 relief covered, near the Columbia at least, by gravels of supposed 

 Satsop age. 



Following the deposition of these gravels came the final uplift of 

 the Coast Range of Oregon and the extrusion of andesites and rhyo- 

 lites forming the superstructure of the Cascade Range and the 

 later development of the volcanic cones now dominating the crest 

 line. Climatic changes that had been presaged by the cooler 

 Pliocene climate culminated in the formation of glaciers on the 

 higher peaks of the Cascades, Stein, Blue, and Wallowa mountains. 

 These glaciers soon extended far down their valleys. The cooler 

 and moister climate permitted the development of extensive 

 lakes within the now semiarid region of eastern Oregon. 



The life of the earhest Pleistocene is best interpreted from the 

 rich avifauna of Fossil Lake and the marine fauna from a few very 

 local embayments bordering the coast. 



The disappearance of most of the Pleistocene glaciers, the 

 deepening of river canyons, the development of alluvial deposits, 

 including auriferous gravels, and the continued volcanic activity 

 include the salient features of late Pleistocene and Recent time. 



IGNEOUS ROCKS 



Igneous rocks are found conspicuously developed in three 

 regions in Oregon: (a) the Blue Mountains, (b) the Cascades, 

 and (c) the Klamath Mountains. In the Blue Mountains the 

 dominant type is granodiorite, in the Cascades basalt and andesite, 

 and in the Klamath serpentinized peridotite, gabbro, and grano- 

 diorite. 



