The worlds largest deposits of oil shale occurs in the Green River Formation of Wyoming, Utah, and Colorado. In Wyoming the Green River Formation consists of rocks which were deposited in and adjacent to Eocene Lake Gosiute. Lake Gosiute fluctuated considerably in size and depth throughout its 4 million year history. It is believed that these fluctuations were climatically controlled. Thus, during the most humid times the lake reached its maximum extent, whereas during the most arid times the lake probably dried up completely. The Green River Formation in Wyoming is generally divided into the Tipton Shale Member, the Wilkins Peak Member and the Laney Shale Member. The Wilkins Peak Member is generally accepted as representing a complicated minimum sand of Lake Gosiute, whereas the Tipton and Laney Shale Members are thought to represent maximum stands of the lake. The richest oil shale in the Bridger Basin occurs in the Tipton Shale Member and the Wilkins Peak Member, whereas the richest oil shale in the Washakie Basin occurs in the Tipton Shale Member and the Wilkins Shale Member. The Green River oil shales (composed of a mixture of alkaline earth carbonates and organic constituents) yield 65-70% of their organic matter as oil (Smith and others, 1959). Culberson (1964) estimated that there are over 370 billion barrels of shale oil in the richer shales of the Bridger Basin alone.