MOHAWK LAKE BEDS. 
387 
and deepening of this stream. If we seek for the cause of 
the former existence of the lake it will be found to be con¬ 
nected with volcanic outflows that occurred about the close 
of the Pliocene or in early Pleistocene time. 
Judging from the present contours of the surface of the 
bed-rock formations (granite and the auriferous slate series), 
the middle fork of the Feather Fiver at the present time fol¬ 
lows approximately an older drainage system, which existed 
before the volcanic outflows, though perhaps not for a great 
length of time. These outflows, largely andesitic breccia 
and tufa, filled up the canon that then existed, and the 
waters thus dammed back formed the Pleistocene lake here 
treated of. Since that time the river has cut through the 
barrier, and the lake has been drained. For three miles 
north of the Mohawk lake beds the Feather Fiver at the 
present time flows through a canon, the walls and bottom of 
which are composed entirely of andesitic and some later 
lavas. 
Pebbles of andesite and of rhyolite abound in the lake beds, 
and, to the north and east of Mohawk Valley, thin patches of 
lake deposit may be seen at many points to rest upon the 
andesite. It is therefore certain that the lake attained its 
maximum development after the andesitic eruptions. 
The lake at its highest level not only filled Mohawk Val¬ 
ley, but extended east into Humbug Valley, having a surface 
of, approximately, thirty-five square miles. The deposits at 
the highest stage were largely rather fine material, andesitic 
and morainal detritus, which has since been much eroded, 
particularly on the east side of Mohawk Valley, where well- 
defined terraces are not to be found. 
But in the northwestern portion of the lake area, at an alti¬ 
tude of about 5,000 feet, the terraces are well preserved. 
The road from Mohawk Post Office to the mining town of 
Johnsville passes over some of these terraces. They are now 
heavily wooded. Exposures of them at various points show 
them to be composed of loose sand and gravel distinctly 
stratified, the bedding being approximately horizontal. 
