Lake Uinta formed when two smaller fresh-water lakes, one in the Piceance Basin and one in the Uinta Basin to the west, expanded during the Long Point transgression (represented by the Long Point Bed) and coalesced to form a single large lake. Salinity in Lake Uinta progressively increased, first killing off the fresh-water mollusks that were ubiquitous during the earlier fresh-water phase and ultimately resulting in the deposition of dolomite-rich oil shale, nahcolite, and halite (Johnson, 1985).