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water of this height would furnish a pressure of 108 pounds to the square
inch. The pressure gauges located in the tunnel, the floor of which is 13
feet lower than the lowest part of the channel, usually registered about
98 pounds, but occasionally as high as 107 pounds. It is improbable that
they registered the full pressure because of relief of pressure owing to the
flow of water from the channel into the mine and because of leakages in
the bore-holes. It was frequently noted during the operations that when a
copious flow of water was obtained the pressure decreased for a time. If
the pressure came from a column of water extending up to what was sup-
posed from the drilling records to be the base of the boulder clay, 83 feet
from the surface, it would have amounted to only 88 pounds to the square
inch. Moreover, it was shown by the sinking of the gravel shaft that no
impervious bed of boulder clay existed at this point down to a depth of at
least 84 feet, although the presence of boulder clay was apparently shown by
the bore-holes north of the shaft. The new tunnel driven from the gravel
shaft south to bedrock at the mouth of Nelson creek is said to have passed
through a bed of boulders and wet, muddy gravel all the way. - The surface
gravels and boulders north of the creek are mostly morainic materials.
South of the creek they consist of Recent stream deposits and tailings.
They are underlain by silty glacial gravels in places, but do not extend
across the valley. The great bed of material, 150 feet thick in the central
part of the valley, referred to in the drilling records as clay, is judged by
the present writer to be glacial stratified silt and clay (slum) because of its
uniform character and because of the absence of stones or boulders. It is
evidently water-saturated, for it was found impossible to sink a shaft, but
probably the downward passage of water is very slow. It seems doubtful
whether the water pressure in the channel was communicated through it.
The basal gravels, however, extend for some distance up the bedrock sides
of the valley and the water pressure may have been communicated through
them. The pressure was evidently due to the height of the column of
water and not to the weight of the overlying materials, for, if the materials
had tended to settle, the pressure would have been greater. The real
difficulty in draining the ground was probably due to the fact that the boul-
der clay does not extend across the valley, for no serious difficulty was met
with in draining the ground at the La Fontaine mine on Lightning creek,
where boulder clay does extend across the valley. It may be that the basal
gravels are very porous and that, therefore, the hydraulic gradient inthem is
very low. It is generally assumed in mining operations of this character
that the amount of ground to be drained is that occupying an inverted cone,
the volume of which depends upon the height of the water column in the
ground (beneath an impervious stratum, if one exist), and the inclination
of the sides of the cone or the hydraulic gradient. Ordinarily the amount
of ground to be drained is not very great, and even in the present case there
seems no reason why the pumping operations should not have been success-
ful unless the basal gravels are porous and are supplied with water from the
surface. A fact which seems to show that water does have access to the
gravels is that in August, 1923, the surface run-off from Slough Creek basin,
as measured by a weir at Slough Creek mine, averaged only one-sixth of the
precipitation over the basin, whereas in other basins in the area it averaged
one-quarter to one-fifth. There is no way of determining what quantities