The Geological History of Fossil Butte National Monument and Fossil Basin

Imagine stepping into a warm, humid wilderness that covered the southwestern corner of ancient Wyoming fifty million years ago. A trio of freshwater lakes—Lake Flagstaff, Lake Gosiute, and Fossil Lake—filled a basin bordered by towering palms, maples, and oaks, while rushes swayed along their shores. Beneath the canopy, early mammals, primates, crocodiles, and flocks of flamingos roamed, and the waters teemed with primitive perch, herring and sting‑ray relatives. When fish died in sudden die‑offs, they sank into soft sediments, beginning a process that would lock them in stone for future discovery.

Today that verdant landscape is a semi‑desert of sagebrush, yet the stone layers of Fossil Butte National Monument preserve the story of Fossil Lake’s rise, fluctuation, and eventual disappearance. This work walks listeners through the geology of the Green River and Wasatch formations, the history of early scientific surveys, and the remarkable fossil fish beds that gave the site its name. By tracing sediment layers and ancient ecosystems, it reveals how a vanished lake can still speak to us across millions of years.