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Fossil Butte National Monument

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Fossil Butte National Monument contains within its boundaries, a significant portion the world's best preserved and most complete ancient lake deposit, known as the Fossil Butte and Angelo Members of the Green River Formation (Oriel and Tracy 1970). Fossil Lake, the Eocene depository of these members of the Green River Formation, occupied a north-south trending structural basin within the Wyoming Thrust Belt known as Fossil Basin. Within Fossil Basin one can follow individual time horizons or rock units from lake center to margin with relative ease and great accuracy. Those rock units have faithfully recorded lake center to lake margin differences in lake depth, turbidity, salinity, alkalinity, temperature, and faunal elements. Areal changes in thickness and lithology (rock type) can be measured and mapped revealing details of lake shape, geography, and regional differences in lake depth and chemistry. Details concerning location and size of fluvial inlets and deltas, variations in lake bottom gradient and sediment type, and how these relate to flora and fauna can be discerned. An understanding of these aspects of the ancient lake system are essential if paleontologists are to put together a complete picture of the lake's fish fauna, which is the focal point of the monument. A variety of basin analysis maps derived from data collected in the field and laboratory will be used to make paleoenvironmental and paleogeographical interpretations of Fossil Lake through time.