GLACIERS OF THE SIERRA COSTA MOUNTAINS 53 
the peaty accumulations about the borders and on the bottoms of 
the lakelets show that the glaciation has not just terminated — 
the ice has been completely gone for at least several thousand 
years. Yet the many lakelets held behind frail barriers of till, 
the cascades and rapids, and the generally uneroded condition 
of the drift tell, in unmistakable terms, of the comparative 
recency, geologically speaking, of the glaciation. Subaerial 
erosion, aside from one main stream channel in each valley, has 
been practically nothing. Even the excavation of the single 
central canyon was largely accomplished while the ice yet lin- 
gered in the heads of the valleys, and by its rapid melting greatly 
increased the streams. With the steep declivities and the heavy 
annual precipitation, it is remarkable how little erosion has been 
accomplished in northwestern California since the glacial epoch. 
Certain cemented river gravels in the valleys of the East Fork of 
Trinity River, the main Trinity River, and lower Coffee Creek, 
which represent the outflow from the glacier, rest upon the lowest 
bedrock in these valleys, and the canyons since excavated in them 
are quite insignificant. Glaciation was one of the very latest 
events in the northern California valleys. That it was of late 
Quaternary age requires no argument. 
The beautiful sky-blue till of the Swift Creek valley has a 
freshness which may be likened unto that of the Wisconsin drift- 
sheet in the Mississippi basin, and oxidation of its surface portion 
has not proceeded to any greater depth. Indeed, the youthful 
appearance of the whole series of glacial phenomena is identi- 
cal with that which has come to be associated in my mind with 
the Wisconsin drift sheet. J am certain that this Sierra Costa 
glaciation was not the age equivalent of the Iowan or any earlier 
drift sheet. I am equally as certain that the glaciers disappeared 
a sufficient length of time ago to carry the glaciation back to the 
Wisconsin epoch. If there were two Wisconsin glaciations in 
the Mississippi basin, as some glacialists seem inclined to con- 
clude, this California glaciation represented the later. At any 
rate, the glaciers. of the Sierra Costa Mountains certainly were of 
Wisconsin age. 
