The formation and location of gold deposits in California has been an important aspect of the location and extraction of gold in native state or gold-bearing minerals. In the early days of the Californian history all the gold was obtained from the alluvial deposits, and after these became exhausted and the deep gravel mines were attacked, attention was drawn to the quartz mines. The shallow placers were formed at a more recent period than the deep placers, which were deposited by an entirely different river system from that now in existence. These ancient rivers evidently flowed at much higher level, and frequently at right angles to the valleys of the present day.
The beds of these rivers belong to the Tertiary age and are most likely of the Pliocene epoch. Their apparently unlimited deposits of metallic gravels were worked on giant scale by an hydraulic process. It is susceptible of proof from several well established facts that at the close of the geological epoch, just prior to the appearance of man upon the earth, the whole of the eastern slopes of the Sierra Nevada Mountains were, below a certain horizon covered by a vast spread of alluvium, owing its origin, probably to the action of extensive glaciers, which have left the evidence of their former presence everywhere in the higher Sierra. The glaciers furnished the transporting power that brought from above the fragments which by long-continued action of running water were worn into the smoothly rounded boulders, gravel and sands forming the gold bearing alluviums.
It has been noted that the melting of the glaciers as their lower skirts reached the warmer zones furnished the water for these ancient rivers, whose beds are now found far above the level of the present river system, and whose courses were generally crossed by the valleys of the modern streams. This condition of things continued long enough to permit the accumulation of beds of gravels, the gold bearing alluvium, to a depth and extent unknown anywhere else in North America; and if we speak of auriferous deposits, unequalled elsewhere in the world. Of the thickness of this accumulated material there is evidence in several places where it has been protected from the action of subsequent denudation by a capping of volcanic materials. In many such places it reached a thickness of 150 m. usually, it has been denuded to one-half of this thickness, often less and in many regions was swept completely away.
Subsequent to the glacial and alluvial epoch to which the gold-bearing gravels are referred, there was a period of intense volcanic activity, the evidence of which is seen most conspicuously in the table Mountains of California, which are capping of basalt forming highly characteristics ranges which portions of the ancient gold bearing gravels were extensively explored in Tuolunine County by tunnels driven beneath the basalt into the ancient river beds. Following the outpourings of the volcanic matter there has been an epoch of very active denudation by running water, which has broken up and removed the volcanic capping, leaving them entire only here and there, as landmarks showings the ancient levels and sweeping away also vast areas of the old alluvium and redistributing it as secondary or shallow placers at lower levels. This denudation was probably consequent on the sudden disappearance of the vast system of glaciers which up to that time crowned the entire range of the Sierra with ice. It was greatly more energetic in the southern portions of the Sierra than in the northern where the mass of ancient alluvium remaining is much greater than it was in the former region. The extent of the ancient alluvium as well as the energy of the power which produced it originally and subsequently denuded it, was determined by geological studies.