The Manix Basin, a structural depression in the central Mojave Desert, bounded by the barren Calico, Paradise, Alvord, Cady, and Newberry Mountains, is the third and lowest major valley of the Mojave River. This withering exotic stream rises in the San Bernardino Mountains, some 200 km to the southeast, and generally carries only a small flow of subsurface water into the Basin. Exceptional storms in the headwater area generate flood flows that occasionally fill the Mojave River channel out to and beyond the town of Barstow. However, surface flow at Barstow is rare.
Some 400,000 – 500,000 years ago, factors relating to elevation and drainage patterns, annual precipitation, mountain snow pack, cloud cover and evaporation were such that the Manix Basin became the site of a freshwater lake known as Pleistocene Lake Manix. The size of the early lake, fed by the inflow of a much enhanced Mojave River, is uncertain, and the early lake is known only by local exposures of its sediments. Lakes persisted in the basin until Late Pleistocene time. The last stand of the Pleistocene lake left a shoreline at an elevation of 543 m (Meek 1989, 1990), indicating a surface area of approximately 236 km2, and a volume of approximately 3.15 km3. Lake Manix drained to the east, perhaps catastrophically, approximately 18,100 years ago, probably as a result of tectonic movement on the Manix fault or a major increase in river inflow that caused the lake to overflow and wash out its topographic dam (Meek 1989, 1990, 1999).
Evidence of the history of Lake Manix remains in the clay, silt, sand, and gravel sequences of the Manix Formation, which contains remains of numerous Rancholabrean animals ranging in age from approximately 20,000 years to well in excess of 350,000 years before present (Jefferson 1968, 1985a, 1985b, 1987, 1989, 1991). The richest fossiliferous section has been well dated by radiocarbon dating, uranium-series techniques, and trace element correlation of a volcanic tephra to a source well dated by the potassium-argon (K/Ar) method. Among the fossils recovered are camel, horse, mammoth, ground sloth, saber-tooth cat, dire wolf, short-faced bear, coyote, flamingo, pelican, eagle, swan, geese, mallard duck, ruddy duck, canvas-backed duck, double-crested cormorant, grebe, crane, seagull and stork. This fauna would have been a bountiful resource for any humans in the vicinity of the lake.
Three separate associations of lithic artifacts are recognizable in the Manix Basin. These are described in the following section.