The majority of gold-bearing gravels were laid down in stream channels during the Eocene and Early Oligocene.   During the Oligocene, Miocene, and Pliocene, volcanic activity in the same region covered some of the auriferous gravels with deposits of rhyolite, andesite, and latite.

In particular, widespread andesitic mudflows and conglomerates were deposited during the Miocene.  These attained a considerable thickness, varying from more than 3,000 feet along the crest of the Sierras to 500 feet in the foothills.  The volcanic flows were so extensive that they almost completely buried the bedrock landscape of the northern Sierra Nevada mountain region.

Over the course of time, rivers carved deep channels up to a couple of thousand feet below the level of the prevolcanic gravels.  This allowed Gold Rush miners to reach the auriferous gravels by digging horizontal tunnels into the sides of the channels.  The advanced stone tools found in these tunnels could be from Eocene to Pliocene in age.  California State Geologist J. D. Whitney concluded  that modern man existed in California previous to the cessation of volcanic activity in the Sierra Nevada.