Several participants in the paleobotanical dig prepare to hike to the fossil plant locality in the Middle Eocene auriferous gravels (usually considered at least in part a correlative geologic time equivalent of the plant-bearing Eocene Ione Formation) in the western foothills of the Sierra Nevada, Nevada County, California--including group leader geologist David Lawler, seen standing in the back of the pick-up truck.
After meeting at Dennys Restaurant in Grass Valley by 9:00 in the morning, members of the FarWest Geoscience Foundation headed east in caravan style several miles to Sailor Flat, an abandoned hydraulic mine that yielded vast quantities of gold during the mid to late 1800s. At a convenient parking area above the old gold diggings, we consolidated forces and piled into several four-wheel drive vehicles for the final assault, a rough and sharply descending rocky ride over a primitive jeep trail to the bottom of the great hydraulic pit.
We were after an unusually thick bed of the fossiliferous, world-famous chocolate-colored shales that are interbedded with the coarse, fluviatile (river-deposited) auriferous gravels--pebble to boulder-sized debris left behind some 50 to 40 million years ago by the Tertiary Yuba River Channel. In swampy, ponded areas along that ancient river channel, quiet-water lacustrine (lake) shales developed on the Eocene landscape and helped preserve an astounding variety of Early Cenozoic Era vegetation. Nearly 70 species of ancient plants have been described from what paleobotanists call the Chalk Bluff Flora, or those associations of fossil plants found in the Middle Eocene auriferous gravels exposed by hydraulic methods during the Gold Rush days of the mid to late 1800s--an awe-inspiring fossil flora whose overall composition resembles a modern subtropical Mexican Elm-Liquidamber forest at the foot of Mount Orizaba in Vera Cruz, Mexico. There are also similarities to such modern subtropical forests as those found along the Rio Moctezuma at Tomazunchale, Mexico; the Liquidamber-Oak and Mexican Elm forests near Coban, Guatemala; and the Liquidamber forests in the state of Morelos and the eastern Sierra Madre west of Tomazunchale, Mexico.