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weathered bedrock beneath unweathered glacial drift of the Pleistocene
period indicate that much of, if not nearly all, the placer gold was freed
from the bedrock and was concentrated in the old stream channels of the
Tertiary period.

During parts of Pleistocene time, valley glaciers were extensive in the
region and accomplished considerable erosion of the bedrock, especially
by cirque action near the heads of the streams and by overdeepening in
places the broad valleys of the main streams, whereas the narrow, V-shaped
valleys became filled with glacial drift and were only slightly eroded. At
other times one or more ice-sheets covered all the area except possibly a
few of the highest points. The ice-sheets appear to have had the effect in
the main of protecting the region from erosion, for they were nearly stag-
nant and transported little drift from one part of the region to another.
There is evidence of one interglacial period during which the glaciers
almost entirely disappeared and considerable erosion of the drift deposits
took place. Gold placers were formed in a few places, mainly by concen-
tration from the glacial drift, for the bedrock was not eroded to any great
extent. The interglacial period was followed by development of valley
glaciers, and possibly also an ice-sheet, although there is no very definite
evidence as to how extensive the later glaciers were. Valley glaciers
extended far down the valleys in late Pleistocene time and finally dwindled
to the very small remnants existing in the district. The presence of large
quantities of stratified glacial sands and gravels shows that powerful streams,
formed from the melting snow and ice, existed at various times, and the
presence of evenly stratified glacial silt and clay at high levels shows that
glacially dammed lakes also existed. The preglacial or Tertiary gravels
in the stream valleys were mostly reworked and incorporated in the glacial
drift, so that Pleistocene placers were formed, though the gold in them was
mostly derived from the older placers. During the closing stage of glacia-
tion the drift deposits were terraced in many places by ice-border drainage
along the sides of the valleys. This action also caused some slight con-
centration of placer gold.

Since the disappearance of the glaciers (except for some small remnants)
a few thousand years ago, the streams have eroded the drift deposits, and,
to a small extent, the bedrock. In the valley bottoms the erosion has
resulted in steep-sided rock canyons, some of which are more than 100 feet
deep, but in the uplands the erosion was only slight. The broad valley
bottoms have been aggraded in places and degraded and terraced in other
places to form nearly level alluvial flats; a soil has been developed and
extensive deposits of peat have been formed. Some concentration of placer
gold has been effected, especially in places where the streams have cut
down through the drift to, or nearly to, bedrock. Weathering of the
bedrock and gold-bearing veins, though only slight, has freed some of the
gold, which is transported by soil creep for short distances down the
hill-sides and affords to the prospector a means of locating gold-bearing
veins that are in many places covered only with small thicknesses of soil
and glacial drift. Earth and loose rock slides have occurred at many
places in the area, especially in the narrow, steep-sided valleys, but are not
prominent features, for few of the valley slopes are very steep and the
mantle of thick vegetation effectively checks earth slides.