PIPER FLORA OP THE STATE OF WASHINGTON. 25 



In some of the John Day rocks remains of plants and animals 

 are found which long since have become extinct. These belong to 

 Miocene-Tertiary time and indicate the existence of a rich and varied 

 flora, strikingly different from that which occupies the region to-day. 



The surface of the Columbia River basalt averages about TOO meters 

 in elevation. Subsequent to its formation occurred the uplifting of 

 the Cascade and Blue mountains. Since that time the geological 

 history of the region has been mainly one of erosion. 



Owing to the soft character of the basalt the principal rivers have 

 worn great canyons in it along their courses. Thus Snake River 

 where it enters the State flows in a tremendous gorge 600 meters 

 deep, this gradually lessening to the westward. WTiere this river 

 cuts through the Blue Mountains between Washington and Idaho it 

 exposes 1,300 meters of basalt rock, which is supposed to represent 

 sibout the original thickness of the combined lava overflows. 



The Columbia River for a great portion of its course has followed 

 close to the line of contact between the basalt and the older granitic 

 rocks. In places its canyon is almost as impressive as that of Snake 

 River. 



Besides the canyons now occupied by streams, there are many 

 others, the abandoned courses of ancient rivers, called coulees. The 

 most notable of these are Grand Coulee (PI. IV) and Moses Coulee in 

 Douglas County, 200 to 300 meters deep. These two coulees were 

 originally enormous cracks in the basalt, and have since been greatly 

 eroded. The bottom of the first named is occupied by a nearly contin- 

 uous chain of lakes. 



The glacial period has left but small traces of its work in eastern 

 Washington outside of the mountains. During this time the canyons 

 of the Snake River and the Columbia were filled to a depth of about 

 100 meters with gravel, most of which has since been removed. The 

 upper Spokane Valley is, however, still composed of such glacial 

 detritus. No evidence of glaciation exists, however, on the surface of 

 the basalt plateau, excepting in the northern part of Douglas County, 

 where glaciers crossed the Columbia, thus blocking it and causing 

 the waters to find a temporary new channel through the Grand 

 Coulee. 



Apparently the gorge of the Columbia River through the Cascade 

 Mountains was blocked at this same time, resulting in the formation 

 of a great glacial lake, called Lake Lewis. This seems to have occu- 

 pied practically the same area as its ancient predecessor, Lake John 

 Day. Except for ill-defined beaches at an elevation of 420 meters and 

 occasional erratic bowlders scattered over Yakima and Douglas coun- 

 ties and doubtless dropped by icebergs, there is little left to show the 

 existence of this lake. 



