A unique bed containing abundant fossil fish, plants, and other fossils occurs within the dominantly unfossiliferous, evaporite-rich, Angelo Member of the Green River Formation. Several hypotheses are considered to explain this unique depositional sequence: the occurrence of a lateral salinity gradient within Fossil Lake (fresh on the margin, hypersaline at the center); a short lived, wetter climate episode; or the development of a marginal freshwater lagoon.

Fossil fish were abundant at both lake margin and lake center locations, suggesting that the lake was fresh throughout its extent. The occurrence of calcite in this unit is also consistent with a freshwater interpretation. These observations preclude a salinity gradient or marginal lagoon for Fossil Lake during this time. This differs from other Angelo Member fish beds that show lake margin to lake center gradients in fossil fish occurrence and lake chemistry. This unique sequence represents a dramatic excursion back to a wetter and more humid climate that characterized earlier phases of Fossil Lake.