A major consideration is the effects of glaciation on the topography of British Columbia in general, and on placer deposits in particular. Moving ice has a tremendous eroding and carrying power and any placer deposit disturbed by moving ice is likely to be dispersed, with the gold mixed with the rest of the glacier’s debris.
In the last several million years most of British Columbia was covered probably by four major ice advances separated by three warm or “interglacial” periods during which the ice melted or “retreated” almost completely. During the build up and retreat the ice directions were greatly affected by local topography and by local glaciers or ice caps from areas of high ground, such as in the Cariboo Mountains.
In general, if the ice moved along a placer valley, the gold and everything else is likely to have been cleaned out and dispersed. If the ice direction was at right angles to the placer valley, then the valley might have been filled with stagnant ice or glacial load which protected the placer from ice erosion. In this instance, the placer may have been substantially unaffected or buried under glacial deposits.
When the ice melts, the debris is dumped as a blanket of glacial till over the countryside. Till consists of a mass of various sizes ranging from large boulders to very fine clay. Pebbles are rounded and boulders may have a characteristic shape with flat faces and rounded edges. Mineralized float or gold particles in till must be traced by following back along the local ice direction, which is usually different from the slope direction. Ice directions can often be identified on air photos and sometimes on topographic maps.
Boulder From Glacial Till
During ice advance and retreat, local drainages are badly disrupted by ice dams or glacial debris. Sometimes old channels are blocked and new ones followed, but usually the old drainage patterns re-establish themselves as the ice retreats. In some places, gold dispersed in glacial deposits is now being re-concentrated into placer deposits. Generally, however, there has been too little time since the last ice retreat for very much concentrating action and such placers are usually rather erratic and of poor quality. The prospector should always be on the lookout for an old or buried river channel. Several placer operations have recently started up on these so-called Tertiary Channels – river channels that existed before glaciation but have been abandoned or buried as a result of glaciation.
Since valley bottoms in most Designated Placer areas are currently heavily staked, old channels or benches may be the best prospecting avenues.