Gravel deposits are an important source of gold and the mining process is called gravel mining or washing, which is performed to obtain placer gold, platinum or diamond. Basically, they are found during the search for precious metals. When the water used in the process is applied under pressure, the term hydraulic mining is applied to the operation. It is important to mention that the deposits operated upon may be glacial drift, the beds and hillsides of modern streams that are traditional placers, and the operation placer mining; or the gravel accumulated in the beds of ancient streams, long since dry, and not infrequently located high on the mountain sides above all the present rivers, may be treated by hydraulic methods when no covered with a lava cap; or drifted by means of tunnels, when the cap of the mountain is too hard to be removed economically, or the top gravel too poor to pay for washing, or water too scant in supply or with too small a head of pressure to admit hydraulic operation.
Basically, the working of ordinary placers by ground sluices or by sluices boxes is usually used. All accumulations of gravel and sand have been made by action of water, either as running streams or sea waves, or by ice as glaciers and icebergs, and result from the wearing away of rocks by the action of the air or rain or frost, or all combined. The nature of their contents must therefore depend on the character of the rock which have been destroyed and width, depth and velocity of the streams which carried the material to its resting place, or the area over which ice sheets and glaciers or floating ice could carry its burden of soil and rocks before melting and depositing its load.
The minerals are disseminated through a variety of rocks, and the presence of veins is not necessarily, as is so generally supposed, to feed the stream. When a stream ceases to show gold in the bed on following it upward, it does not follow that there must be a rich vein in that vicinity, for which gold prospectors try to find gold. It may be the upper limit of the gold bearing rocks. It is possible to find good placers diggings in regions where well defined veins are scarce. If the distribution of gold in certain belts of rock is found, the origin of gravel deposits carrying gold becomes much more easily intelligible.
When the rock is decomposed by the ordinary action of frost or air and rain, the detached particles are constantly descending by steepest line under the action of gravity of water, and this is usually more or less at right angles to the stream into which they finally find their way. During this descent the particles of gold suffer, but little abrasion, as their progress is excessively slow, and they undergo no sorting into sizes, other than what would result from the greater momentum of the larger particles causing them to travel further on a steep side where little surface water and rivulets acted, but is there is much water coming down the reverse might occur, the larger particles resisting more than the light flaky ones.