British Columbia placers have formed largely as a result of stream concentration. The disintegrated material from a gold-bearing lode gradually moves downhill under the influence of slope wash until it reaches a stream channel. The sorting action of running water results in small fragments and material of lower specific gravity being moved selectively downstream. In addition, the material in the stream bed is continually being agitated so that the minerals of high specific gravity tend to work down, eventually becoming concentrated on the creek or river bottom. Nuggets of gold sink rapidly through the gravel without travelling far downstream. Finer gold is gradually and continuously carried farther, and very fine flaky gold may be carried many miles downstream.
During its life history a stream may pass through a number of’ stages that affect the formation of placers. Where a stream is down-cutting, bedrock is being scoured and any gold lodged on it will move downstream. In other sections of the stream, flattening of the grade, overloading of the stream, or slackening of the current on the inside of a bend will cause gravel and gold to be deposited. In some sections, the stream may swing from one side of the valley bottom to the other, eroding on the outside of a curve and depositing material on the inside. Bars form in large creeks during certain stages, and fine gold that is carried by flood-waters is deposited in them.
All streams have a sorting action upon the gravels in their beds, but extremely rich placers are formed only by the concentration of gold released from lodes during long periods of weathering and stream erosion.
In British Columbia, active down-cutting by streams took place in late Tertiary or early Pleistocene time, with the result that stream valleys were deepened, leaving rock benches, on some of which the gravel is rich enough to be mined profitably.
The formation of British Columbia placers has been greatly affected by glaciation. Normal development of placers in many mountain streams has been greatly modified; in numerous places gravel has been dispersed, bedrock eroded, and a layer of boulder clay deposited by valley glaciers. Overloading of streams and disturbances of drainage brought about during interglacial and postglacial times have complicated both the formation of placer deposits and the search for them.
In the early stages of glaciation, before there was much accumulation of ice, there was undoubtedly an increased precipitation and a high run-off during the summer months. This run-off swept much of the weathered rock debris and residual gold deposits into the valleys. The valleys were filled in many instances to depths of several hundreds of feet with this material. Many streams were unable to transport the tremendous load supplied to them, with the result that previously formed placers were buried to considerable depths.
During interglacial periods the streams cut down through these deposits and in places cut through rock-spurs projecting into the old valleys. Since the final retreat of the ice, streams have again cut down their channels in the filled valleys and locally in bedrock. Most present-day streams follow old valleys, in part at least, but in some instances streams have completely abandoned their former valleys.
Stream activity during interglacial periods and in recent times caused the erosion of some earlier placers and their re-deposition as placers of various ages, origin, and richness. Old gravel-filled channels, possibly cut through and partly destroyed by later streams in a valley, may still remain as bench deposits, deep leads, or buried channels, higher or lower than the present stream bed.
Postglacial streams have reworked older gravel and boulder-clay deposits to form surface enrichments that extend down to only shallow depths.