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Lightning creek is over 2,000 feet wide. The stream flows in a compara-
ively narrow, drift-filled valley for 2? miles up from Lightning Creek flats.
At the Mathers shaft, three-quarters of a mile upstream, the valley is
somewhat constricted by a ridge of hard rock which outcrops on the east
side. A short distance above the Venture shaft, 22 miles upstream, the
creek flows in a shallow rock canyon on the west side. Drift, morainic
ridges, block the main valley for about one-quarter mile. A small stream,
at the mouth of which some mining was done in the early days, comes in
from the northeast one-quarter mile above the Venture shaft, and 14 miles
farther up the main stream is joined from the southwest by Campbell creek,
on which considerable mining was done, especially in the lower part where
the ground is shallow and was mined to bedrock by open-cuts. In the
upper part some work was done in 1922 and 1923 by Paul Barnette and
William Slade who obtained about $200 in gold, including a 5-ounce nugget,
by ground-sluicing the surface gravels. About three-quarters mile up the
creek, they attempted to sink a shaft through boulder clay to bedrock, but
a heavy flow of water came in at the 35-foot level, when the shaft had
reached a depth of 42 feet, and the work was abandoned. Higher up
Peters creek, Carruthers and Basford (Basswood) creeks, both of which
have been mined to some extent, come in from the northeast. The fact
that several of the tributaries of Peters creek were known to be gold-bearing,
at least to some extent, has led to several attempts at mining the deep
channel of Peters creek. The ground on the creek is shown by borings and
shafts to be at least 125 feet deep where the stream issues into Lightning
Creek flats, 70 feet at the Mathers shaft, 35 feet at the Venture shaft, and
37 feet three-quarters of a mile farther upstream. The surface grade of the
creek from the Venture shaft to Lightning Creek flats, a distance of 15,000
feet, is 2-15 per cent. The average grade of the bedrock channel from the
Venture shaft to the Mathers shaft is 2-3 per cent. The average grade of
the bedrock channel from the Mathers shaft to the bore-hole near the
mouth is 4-1 per cent, and the ground may be somewhat deeper near the
bore-hole. The bedrock channel of the creek, therefore, is much steeper
in the lower part than in the upper, which suggests that the main valley of
Lightning creek was somewhat over-deepened by ice erosion. ‘The deposits
in the valley of Peters creek consist of surface stream-gravels underlain in
places by clay and in other places by glacial gravels. A peculiar kind of
clay known as ‘‘swelling clay’ occurs on the creek, especially along the west
side from near the Mathers shaft to about 2,000 feet above the shaft. Ter-
races are cut in it above the shaft and the long tunnel starting near the
Mathers shaft was runinit. Similar clay is known to occur at only one or
two other places in the area. In running tunnels through it, the material
had to be shot out a set at a time, for it is fairly hard and quite dry except
near the surface. On exposure to the air in the tunnel the clay swells to such
an extent that the timbering of the tunnel is likely to be broken. After a
few days, the clay ceases to swell and the mine timbers may safely be
replaced, for no further swelling occurs. The clay contains no stones, is not
calcareous, is exceedingly fine grained and plastic, and shows only faint
stratification. The clay swells on exposure to the air because it contains a
great deal of colloidal or very fine-grained material, which enables it to
absorb from the air sufficient moisture to increase its bulk several times.