The trona (hydrated sodium bicarbonate carbonate) beds of Sweetwater County, Wyoming are noted for a variety of rare evaporite minerals.

The Green River Formation, is the type locality for eight rare minerals: bradleyite, ewaldite, loughlinite, mckelveyite-(Y), norsethite, paralabuntsovite-Mg, shortite and wegscheiderite.

The limestone matrix is so fine-grained that fossils include rare soft parts of complete insects and fallen leaves in spectacular detail.

The climate was moist and mild enough to support crocodiles, which do not tolerate frost, and the lakes were surrounded by sycamore ( e.g. Platanus wyomingensis [9]) forests.

The lake system formed over underlying river deltas and shifted in the flat landscape with slight tectonic movements, receiving sediments from the Uinta highland and the Rocky Mountains to the east and north.

Lack of oxygen slowed bacterial decomposition and kept scavengers away, so leaves of palms, ferns and sycamores, some showing the insect damage they had sustained during their growth, were covered with fine-grained sediment and preserved.

Vertebrates were preserved too, including the osteoderms of Borealosuchus, the crocodile that was an early clue to the mild Eocene climate of Western North America.

Besides fishes they include at least eleven species of reptiles, and some birds and one armadillo-like mammal, Brachianodon westorum, with some scattered vertebrae of others, like the dog-sized Meniscotherium and Notharctus, one of the first primates.

The first documented records of (invertebrate) fossils from what is now called the Green River Formation are in the journals of early missionaries and explorers such as S. A. Parker, 1840, and J. C. Fremont, 1845.

[7] Geologist Dr. John Evans collected the first fossil fish, described as Clupea humilis (later renamed Knightia eocaena), from the Green River beds in 1856.

[14] The unusual chemistry of the lakes in which it was deposited makes the Green River Formation a major source of sodium carbonate.