Bottom gravels are the exact counterpart of the modern gold placers and were formed by the alternate flood deposits and slack water panning. Basically, these bottom gravels are usually the only ones which pay for the slow process of drifting, and sometimes, when too poor for this method of working, the entire top dirt has to be removed, even if it barely cover expenses. The first stage was similar to the conditions surrounding current streams. The wearing away of the hills was carried on comparatively slowly; the streams were of moderate velocity, carrying off the waste and leaving the coarse gravel and gold in the river beds.
The large quantities of material brought down in flood time were gradually panned out during the drier seasons, by the thin layers of iron sand similarly to the cleanup of the stream in a river bank and sea beach, leaving the contained gold to enrich the bottom deposits. The quantity of sediment sent down had been so great that the streams at the mouths of the mountain valleys became filled up and the sediments began to accumulate in the valleys themselves, and from that time the material deposited assumed a flat grade and the filling up of the bottom proceeded at a rapid rate, creeping upward as the lower portions became more and more chocked, until it is found the thickness of these beds running up into the hundreds of feet.
The gold in such material may be fine, and not having undergone the process of concentration to such extent as the bottom gravel is much less in quantity for equal bulk of material; but there is not sharply defined line between the two, although the depreciation takes places rapidly until the general average value of the upper fine bottom is reached. This results from the fact that there is to some extent a combination of both conditions at or near the point where the flat portion of the valley met the steeper slope of the mountain stream. In this way, as the valley filled up the river channel naturally became wider, the sheet of water thinner and in consequence less able to carry any burden in suspension and it is easy to picture a wide sandy bottom with changing channels, bars and stagnant pools obstructed by snags.
There are occasional periods of drought when the water was comparatively free of sediment, yet a with a velocity sufficient to pick up and remove the finer sand to a certain extent. This can be noted as thin beds of fine gravel which appear on the face of the bank and which by their superior richness indicate a certain amount of concentration.