Green River Formation
The Green River Formation produces the most beautifully preserved fossil fish in the world. Deposited in a system of large ancient lakes during the Eocene epoch, the fine-grained limestone (oil shale) preserves organisms in extraordinary detail — often with complete skeletons, scales, fins, and sometimes even gut contents visible.
Geological Context
During the early Eocene, three large lakes occupied what is now southwestern Wyoming, northwestern Colorado, and northeastern Utah:
- Fossil Lake (smallest, deepest) — Produces the finest specimens. Located near Kemmerer, Wyoming.
- Lake Gosiute (largest) — Covers the Green River Basin of Wyoming.
- Lake Uinta — Extends into Colorado and Utah.
The lakes periodically stratified, creating oxygen-poor bottom waters that prevented scavenging and decomposition. Organisms that sank to the bottom were buried in fine carbonate mud and preserved in remarkable detail. The result is one of the world’s great Lagerstätten — a deposit of exceptional fossil preservation.
What You’ll Find
Fish (The Stars of the Formation)
- Knightia — The most common fossil fish in the world. Small herring-like fish, 3-6 inches typical. You will find these.
- Diplomystus — A larger herring relative, 6-18 inches. Less common but frequently found. Sometimes preserved eating a Knightia.
- Mioplosus — A perch-like predatory fish. Prized by collectors for its dramatic appearance.
- Priscacara — A sunfish relative with tall, compressed body and dramatic dorsal spines.
- Phareodus — A bony-tongued fish with large teeth. Premium specimens.
- Heliobatis — A freshwater stingray. Spectacular when found complete.
Other Organisms
- Insects — Beetles, flies, wasps, and ants preserved in extraordinary detail
- Plants — Palm fronds, ferns, leaves, flowers, and seeds
- Turtles — Multiple species, occasionally found complete
- Crocodilians — Rare but known from the formation
- Birds — Very rare, but several species described
- Bats — Icaronycteris, one of the oldest known bats, comes from the Green River Formation
- Early horses — Protorohippus (formerly Hyracotherium), extremely rare and valuable
Collecting in the Green River Formation
Commercial Quarries (The Main Access Point)
Several commercial quarries near Kemmerer, Wyoming allow visitors to split limestone and keep what they find:
- Warfield Fossils — Family-run quarry, fee-based digging
- American Fossil Quarry — Fee-based, tools provided
- Ulrich’s Fossil Gallery — Fee-based quarry plus museum and gift shop
- Tynsky’s Fossil Fish — Another fee-based operation
Prices typically range from $75-150 per person for a full day of digging. You split thin limestone layers with provided tools and keep every fish you find. Most visitors find multiple Knightia; Diplomystus and other species are less common but regular.
BLM Land
Some Green River Formation exposures on BLM land allow casual collecting of invertebrate and plant fossils. Vertebrate fossils (fish included) on federal land technically require a permit, though enforcement varies. Check with the local BLM field office.
Fossil Butte National Monument
No collecting permitted. But the visitor center has an outstanding display of Green River Formation fossils, and the quarry site interpretive trail is worth visiting for geological context.
Tips for Green River Collecting
- Thin is good — The finest specimens come from the thinnest limestone laminae. Split carefully and check both surfaces.
- Look for dark outlines — Fish appear as dark impressions against the lighter limestone.
- Humidity helps — Slightly damp stone is easier to split and shows fossils more clearly.
- Bring patience — Splitting limestone is meditative work. Rushing breaks good specimens.
- Wrap carefully — The limestone is fragile. Wrap each slab in newspaper for transport.