Green River Formation
The pale, finely laminated Eocene lake beds of the Uinta Basin — Lake Uinta's marlstone and oil shale, world-famous for fossil fish, in the walls of Desolation and Gray canyons.
Also known as: Green River
Paleogene — mammals take over after the dinosaurs
The world back then
Eocene, about 48 million years ago. This is the bottom of a huge lake — Lake Uinta — that sat here for millions of years, its still water dropping a paper-thin layer of fine mud each season. Fish, insects, and leaves drift down and are buried whole. Warm, calm, and green, the surface stretching to the horizon like a shallow sea that never leaves.
The lake is long gone, but you're floating the river that cut into its bed, reading its still, papery layers as they slide by. Millions of years of quiet water settled here; you get the pleasant afterlife of the place with none of the drowning. Being the creature that inherits the calm version of an old lake is a fine way to spend deep time.
Recognize it in the field
- Typical color
- buff to gray, olive, and brown
- Weathers as
- Ledge- and cliff-forming finely laminated carbonate and oil shale — the pale middle walls of Desolation and Gray canyons.
- Grain
- very fine; finely laminated
Look for
- finely laminated marlstone, limestone, and oil shale
- the world's richest oil-shale deposits (Parachute Creek Member)
- exceptional fossil fish, insects, and plants
- seasonal lake-floor laminations (varves)
How this rock was made
Lake Uinta, one of a chain of long-lived Eocene lakes (with Lake Gosiute) that filled the Green River basins for millions of years.
The setting: lake-bottom (lacustrine) (Lake Uinta)
Up to ~2,000-3,000 ft in the basin depocenter.
In the rock record
↑ Younger — rests above
↓ Older — sits beneath
Where to see it
Regions: Uinta Basin (Colorado Plateaus province)
- the middle walls of Desolation and Gray canyons (Green River, Uinta Basin)
Float past it: Desolation Canyon · Gray Canyon · Uintah Basin