Thursday, 10 July, 2025 / Edition 65

This week, we’ll take a trip to explore the Green River Formation, world renowned for its Eocene fish fossils, and much more. Grab your rock hammer and let’s head to Wyoming! ⚒

Sharon Lyon

Editor, GeoLifestyle

Fossil Butte National Monument, Wyoming

Fossil fish on display at the Museum at Fossil Butte National Monument.

Fossil Butte National Monument preserves the remains of an ancient lake, one of three large lakes that existed in southwestern Wyoming during the Eocene approximately 52 million years ago.

Area geology: The region was a foreland basin during the Laramide Orogeny. Uplift of the surrounding highlands led to erosion and deposition in the lake basin.

• Fossil Butte is a flat-topped ridge, capped with erosion-resistant limestone. The surrounding softer sediments have eroded away, exposing the fossil layers.

• The main rock unit here is the Fossil Butte Member of the Green River Formation. The formation is composed primarily of fine-grained limestone, interbedded with shale and volcanic ash layers.

• The limestone is laminated into varves formed in low-energy water, reflecting seasonal variations in sediment and organic material.

Paleontology: The Fossil Butte Member is considered a lagerstätte, renowned for its exceptional fossil preservation and variety.

• The most common vertebrate fossil in the world, the herring-like fish Knightia eocaena, can be found here. Less common is the extinct species of temperate bass, Priscacara serrata. 

• In addition to fossil fish, the rocks contain stingrays, plants, insects, and more rarely, three-toed horses, crocodiles, birds, bats, and turtles. The oldest complete bat fossils and the only complete freshwater stingrays are found here.

• The fine-grained sediments have even preserved soft tissues, feathers, and stomach contents.

Preservation conditions: As the dead animals’ carcasses settled on the lake bottom, three conditions aided their preservation:

1. A layer of saltwater at the bottom of the freshwater lake kept scavengers away from the remains.

2. Very little oxygen at the bottom of the lake prevented decay.

3. A microbial mat of cyanobacteria on the lake bottom grew over the carcasses, holding the animal remains in place and preventing them from breaking apart.

Trip tips: Be sure to go to the Visitors Center and museum to see the exquisite Eocene fossils. The museum is worth the drive.

• Hike along the four miles of maintained trails. There is no fossil hunting allowed in the monument and the trails do not reach the fossil beds.

Learn more about visiting Fossil Butte here.

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American Fossil, Kemmerer, Wyoming

A fossil slab containing fish fossils of Knightia eocaena, American Fossil, Wyoming.

Digging at American Fossil gives you the opportunity to hunt for these exceptional Green River fossils yourself. And you get to keep all the fossils you find! I had the opportunity to visit American Fossil a few summers ago, and it was so much fun! I shipped home ten boxes of fish fossils (with apologies to my mail carrier).

This week, I reached out to Seth Sorensen, co-owner of American Fossil and owner of Fossil Shack. Here are snippets of our conversation:

What makes American Fossil so special?

Sorensen: “As fossil and geology lovers ourselves, we love to share that passion with our guests. We want everyone to leave with the same level of excitement we have towards this one-of-a-kind location. 

We were the first vertebrate fossil quarry in the world that allowed guests to keep every fossil they find and now have over a decade in making this an experience that our guests will treasure for a lifetime.

We also work closely with museums and research institutions around the world to make sure the science of this formation is studied and curated. We have donated specimens to many museums far and wide, including the holotype specimens for many newly discovered species.”

What has been the most unique fossil found at American Fossil?

Sorenson: “One of only two specimens of Protorohippus (three-toed horse), and the holotypes of all three bat species come from the American Fossil Quarry. Several unique birds, such as parrots, have been found here, large crocodiles and caiman have been found here, and every species of turtle from the formation, including the holotype and paratype of a new species that is currently being described by the Florida Museum of Natural History scientists.”

Do all visitors find fossils?

Sorenson: “Every guest will find a fossil. It is estimated that there are still over 10 trillion fossils buried in the formation just waiting to see the light of day after 53 million years.”

Is someone onsite to help identify the fossils?

Sorenson: “We have a whole team of guides and experts onsite to help identify the fossils.”

When is the best time to visit?

Sorenson: “We are open from Memorial Weekend through Labor Day weekend and possibly a few weeks in September, depending on weather. My favorite times to dig are June and August. Those two months typically provide the best weather.”

Follow: American Fossil on Instagram @american_fossil_quarry

Read more about visiting American Fossil here.

Lagerstätte Library

Some summer reading suggestions. Cover images from Amazon.com.

In addition to co-owning American Fossil, Sorensen is the author of three books:

1. History’s Aquarium is a guide to the fossils of the Fossil Butte Member of the Green River Formation, with detailed photographs.

2. Head Full of Rocks: Mostly True Tales of a Commercial Paleontologist is a collection of hilarious short stories of one man’s adventures in the world of paleontology.

3. The Utah Story shares about the unique stories, events, and people that shaped Utah into the great state it is today.

If you’ll excuse a bit of self-promotion, my novel Fossil Woman, features the Green River Quarry as a destination for my protagonist Henrietta Ballantine, as she studies paleontology in the male-dominated field of the 1950s. The setting was inspired by my trip to American Fossil!

Many thanks to Seth Sorensen for his contribution to this newsletter. Happy exploring, everyone!

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✉️ To get in touch with Sharon, send an email to editorial@aapg.org.