Ione Formation

The formation regionally overlies the Martinez Formation and regionally underlies the Chico Formation.

The auriferous gravels accumulated on slope of Sierra Nevada during the Neocene Period, and at same time there was deposited in the gulf then occupying the Great Valley a sedimentary series consisting of clays and sands.

It is usually overlain by a few feet of reddish Pleistocene gravel and rests on Chico formation. The strata form flat-topped hills, and consists of a succession of light-colored clays and white or yellowish-white sandstones. The largest development occurs south of American River.

• List of fossiliferous stratigraphic units in California
• Paleogene California
• Paleontology in California