Searles Lake is an endorheic dry lake in the Searles Valley of the Mojave Desert, in northwestern San Bernardino County, California. The lake in the past was also called Slate Range Lake and Borax Lake. The mining community of Trona is on its western shore. The evaporite basin is approximately long and at its widest point, yielding 1.7 million tons annually of industrial minerals within the basin to the Searles Valley Minerals mining operations. Searles Lake is bounded by the Argus and Slate Mountains. Named after John Wemple Searles. Geology The stratigraphic record at Searles Lake shows that it once held brackish water as deep as . Fluctuations in lake levels correspond to the advances and retreats of glaciers in the Sierra Nevada Range. Thirty major lake levels occurred during the last 150,000 years, represented by a sequence of salt and mud beds. The precipitation of minerals occurred during long periods of lake evaporation. The lake is home to the Trona Pinnacles, a spectacular geologic tufa formation and a National Natural Landmark. History Borax was first produced from the dry lake surface in 1873 by John Searles under the name of the San Bernardino Borax Mining Company. Searles was the first to haul borax using the famous 20 mule team wagons. In 1873, before the railroad was built to Mojave, refined borax was hauled 175 miles by 20 mule teams from Slate Range Playa (now called Searles Lake) to the harbor at San Pedro. The Searles Lake borax discovery has been designated California Historical Landmark #774, with a plaque at the roadside rest area in Trona. From 1922 to 1928, the Epsom Salts Monorail crossed the Searles Lake on a wooden trestle. Mineralogy Searles Lake is a huge resource of sodium and potassium minerals of the carbonate, sulfate, borate and halide classes of mineralogy. The manufacture of industrial minerals involves a complex solution mining operation in which naturally occurring brines are pumped from wells completed in several salt beds. The brine wells range in depth from near-surface to over below the salt pan. A network of production wells, injection wells, solar ponds and piping are used in the production and treatment of the brines. Industrial minerals are extracted from the brines at the Argus, Trona and Westend plants. Minerals are crystallized from the brines, screened, washed, and dried. The crystals are then baked in rotary kilns to drive off water