lakes; it filled Death Valley during each glacial period from 240,000 years ago to 10,000 years ago. By 10,500 years ago these lakes were increasingly cut off from glacial melt from the Sierra Nevada, starving them of water and concentrating salts and minerals. The desert environment seen today developed after these lakes dried up. Early sedimentation Proterozoic complex Little is known about the history of the oldest exposed rocks in the area due to extensive metamorphism; the rock has been pressure-cooked. This somber, gray, almost featureless crystalline complex is composed of originally sedimentary and igneous rocks with large quantities of quartz and feldspar mixed in. The original rocks were transformed to contorted schist and gneiss, making their original parentage almost unrecognizable. Radiometric dating gives an age of 1700 million years for the metamorphism, placing it in the early part of the Proterozoic eon. A mass of granite now in the Panamint Mountains intruded this complex 1400 mya. Pegmatic dikes and other widely spaced plutons of granite are also in the complex (a pluton is a large blob of magma deep underground and dikes are projections of that). Outcrops can be seen along the front of the Black Mountains in Death Valley and in the Talc and Ibex Hills. When the granite was being intruded, the west coast of North America ran through Eastern California and through an embayment that spread toward the Las Vegas Valley. This embayment, called the Amargosa aulacogen, had highlands north and south of it and was the result of a failed rift. Many thousands of feet of sediment filled the slowly subsiding basin. Next, the metamorphosed Precambrian basement rocks were uplifted and a nearly 500-million-year-long gap in the geologic record, a major unconformity, affected the region. Geologists do not know what happened to the eroded sediment that must have overlain the complex, but they do know that regional uplift was responsible; the area was originally below the surface of a shallow sea. Pahrump Group The Pahrump Group of formations were deposited from 1200 to 800 mya in the Amargosa aulacogen. This was after uplift-associated erosion removed whatever rocks covered the Proterozoic Complex. Pahrump is composed of, from oldest to youngest: Crystal Spring Formation, Beck Spring Dolomite, Kingston Peak Formation. Outcrops of this group can be seen in a highly metamorphosed belt that extends from the Panamint Mountains to the eastern part of the Kingston Range, including