Aerial view of a hillside excavation at golden hour

Green River Formation

UChicago Stones & Bones 2026

Exploring the world 55 million years ago.

LAT: 41° 47′ 42.312″ N
LON: 110° 42′ 26.208″ W
ALT: 2,241.5 m

Present day

55 million years ago

Lush prehistoric lake landscape at Fossil Butte with palms, water lilies, and distant mountains

Fossil Lake

Paleogeography

The smallest of three

In the early Eocene, three lakes filled the Green River Formation. Fossil Lake was the quiet outlier — a compact basin near present-day Kemmerer, Wyoming, west of Lake Gosiute and north of Lake Uinta, separated by the Uinta Mountain uplift.

Map of Eocene lakes Fossil Lake, Lake Gosiute, and Lake Uinta across Wyoming, Utah, and Colorado, with Kemmerer marked near Fossil Lake — Extent of Fossil Lake, Lake Gosiute, and Lake Uinta during the early Eocene. Fossil Lake sits beside Kemmerer, Wyoming — the westernmost of the three Green River basins.

Preservation

Exceptional preservation

Quiet, oxygen-poor water sealed fish in fine mud before scavengers could scatter them — leaving spines, ribs, and forked tails still locked in life position.

Nearly complete fish fossil with articulated spine, ribs, and forked tail preserved in light limestone — A fully articulated specimen from the Green River beds — eye socket, fin rays, and caudal tips intact against the pale limestone matrix.