King’s Systematic Geology of the 40th Parallel. 297 
e plains at the mouths of the mountain valleys. An 
immense amount of erosion was accomplished during the Gla- 
cial epoch and there is evidence, to be given below, that it was 
done during two periods of glacier extension in the Quaternary ; 
during the first and greater the floods cut the deep V-shaped 
cafions, and during the second the glaciers transformed the 
upper part of these into U-shaped cafions, and we may add that 
the second floods deepened the V cafions below the foot of the 
lacier. 
4 The Great Plains are underlaid by beds which were depos- 
ited in a great fresh-water lake. In the southeast these beds 
dip under the Gulf of Mexico, while near the Rocky Moun- 
tains they are 7,000 feet above the Ocean. It is therefore evi- 
dent that they have been tilted, for otherwise we should have 
to suppose that there existed a lake whose surface was 7,000 
feet above the sea, and for which there was no eastern enclos- 
ing wall. Both General G. K. Warren and Mr. King have 
shown that after the Pliocene such a tilting really took place, 
so that during Quaternary time this declivity had, as now, free | 
drainage to the Ocean, and was traversed by the rivers flooded 
from the glaciers. ee 
n the Great Basin the Pliocene—Shoshone—lake was dis- 
were swept by hill-wash and river-floods far out from the parent 
mountains. During their prime, these inland fresh-water seas 
were filled to their outlets, Lake Bonneville, over 1,000 feet 
deep, drained through Red Rock Pass into the Snake and 
Columbia Rivers. The outlet of Lake Lahontan may have 
wea southward, and the lake must have been over 500 feet 
eep. 
