PIPER—FLORA OF 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 700 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. Where this river 
cuts through the Blue Mountains between Washington and Idaho it 
exposes 1,300 meters of basalt rock, which is supposed to represent 
about 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- 
ous 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, 
here 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. 
