The Luman Tongue of the Green River Formation corresponds to the initial impoundment of Lake Gosiute and the transition from dominantly fluvial to lacustrine deposition in the southern part of the greater Green River basin. Luman sedimentary facies consist of calcareous mudstone, fossiliferous mudstone, coquina, sandstone, and coal. We interpret these lithologies to represent deeper lacustrine environments (mudstone and coquina) that grade upward into beach fluvial environments and coal-producing swamps. The vertical organization of these lithologies indicates a minimum of three parasequences and the establishment of a robust mollusk community by the end of Luman time. Lateral facies relationships within the Luman reveal a channel inlet to Lake Gosiute at Telephone Canyon that can be recognized by a localized change from alluvial to lacustrine facies. The paleoenvironmental reconstruction of southwestern Wyoming during Luman time is that of a freshwater lake surrounded by fluvial plains and marshes which prograded periodically, filling in the basin with coal and fluvial sand. In many locatilities, the Luman Tongue is overlain by a cobble conglomerate representing a shift to a high-energy fluvial environment, coincident with the transition into the Niland Tongue of the Wasatch Formation. Within the conglomerate, clast lithologies are dominated by Uinta Mountain Group quartzite, indicating a source from the Uinta Mountains. By examining the character of the Luman Tongue and its contacts with the Wasatch Formation, the influence of climate and tectonics on the Green River Basin can be explored.