THE TERTIARY PERIOD 



303 



intermontane basins much nearer sea- 

 level, like the recent deposits of the Great 

 Valley of California. 



The dissection by erosion of the usu- 

 ally comparatively soft and only moder- 

 ately tilted deposits of Tertiary age in the 

 western interior has given rise to much of 

 the "Bad Lands" country, so called be- 

 cause of the difficulty early explorers had 

 in travelling across that rugged region 

 (Fig. 182). 



Igneous Activity. — In connection with 

 the discussion of the close of the Creta- 

 ceous period, we spoke of the inaugura- 

 tion of igneous activity which resulted in 

 the building up of the vast lava plateau, 

 occupying fully 200,000 square miles, be- 

 tween western Wyoming (including Yel- 

 lowstone Park) and the Cascades and 

 southward into northern Nevada and 

 northeastern California (Fig. 184). Most 

 of this lava was poured out in Tertiary 

 time, particularly in the latter part. 

 Norton has clearly and concisely stated 

 the principal facts as follows: "For thou- 

 sands of square miles the surface is a lava 

 plain which meets the boundary moun- 

 tains as a lake or sea meets a rugged and 

 deeply indented coast. . . . The rivers 

 which drain the plateau — the Snake, the 

 Columbia, and their tributaries — have 

 deeply trenched it, yet their canyons, 

 which reach the depth of several thou- 

 sand feet, have not been worn to the base 

 of the lava except near the margin and 

 where they cut the summits of mountains 

 drowned beneath the flood. Here and 

 there the plateau has been deformed. . . . 

 The plateau has been built like that of 

 Iceland, of innumerable overlapping sheets 



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