46 p. B. KING 



Columbia Plateaus, partly split in their middle by the emerged 

 Nevadan rocks in the highlands of northeastern Oregon. 



Early Tertiary Environments. The record of early Tertiary time 

 may be seen mainly in the Coast Ranges, where marine clastic 

 sedimentary strata several miles thick are turned up in gentle folds. 

 Some of them, such as those of the Olympic Mountains of north- 

 western Washington, may have been laid down beneath ocean water 

 of considerable depth, and have been derived from turbid flows that 

 moved westward down the continental slope. Interbedded with the 

 Tertiary sedimentary rocks are great lenticular masses of basaltic 

 lava, which were largely erupted beneath the sea, as shown by their 

 pillow structure (Waters, 1955a, pp. 204-707). Inland, where the 

 lower Tertiary rocks are occasionally exposed , the marine beds pass 

 into land-laid deposits, including beds of coal, which formed in 

 floodplains, swamps, and lakes. 



During early Tertiary time the northwestern volcanic province 

 probably was a broad coastal plain with an offshore continental shelf 

 and slope, which faced westward on open ocean (Fig. 5). Its environ- 

 ment must have resembled that of the present Gulf Coastal Plain, 

 except for the much greater volcanic activity. No ranges existed 

 near the coast to create a climatic barrier like that today; Eocene 

 floras from both the east and west sides of the present Cascade Range 

 are closely related, and grew in subtropical lowland forests (Chaney, 

 1938, p. 3^Z). 



Middle and Late Tertiary Environments. Miocene time witnessed 

 the great eruptions of Columbia River basalt, which spread over an 

 area of 100,000 square miles, in places to thicknesses of a mile or 

 more (Waters, 1955a, pp. 707-708). The basalt flows cover all the 

 plateau country of southeastern Washington and northeastern 

 Oregon, within the Nevadan arc (Fig. 9), where in many places they 

 still remain nearly horizontal, although in others they have been 

 warped and folded. Along the lower course of the Columbia River 

 the basalt also spreads westward across the site of the Cascade 

 Range. Here its farther edges interfinger with marine deposits. 



The Columbia River basalt is a plateau basalt — it was not erupted 

 from volcanoes, but welled out of deep fissures from the simatic layer 

 beneath. Single flows are 100 to 500 feet thick, and must have been 

 very fluid as some of them are traceable for more than a hundred 

 miles. The flows were probably piled thickest near the center of the 



