Lake Bonneville was a large ancient Pleistocene-era freshwater lake that existed about 32,000 to 14,000 years ago, contained for thousands of years by mountains acting as natural dams in the eastern Great Basin. At its greatest extent it covered more than 20,000 square miles, stretched about 325 miles long and 134 miles wide, and reached depths of just over 1,000 feet as it spread over much of Utah and into Nevada and Idaho under a wetter, colder climate, fed by precipitation and melting glaciers. Waves and erosion cut shorelines and terraces into the surrounding rock, including the Stansbury, Bonneville, Provo, and later Gilbert terraces; the Stansbury shoreline is the oldest of the first three, dating over 30,000 years, and the Bonneville Terrace is the highest at just over 5,000 feet above sea level. About 14,500 years ago, where this area now lies would have been under approximately 530 feet of water, and at that height the lake broke through a natural earthen dam at Red Rock Pass in southeast Idaho and spilled into a tributary of the Snake River in the event known as the Bonneville Flood, lowering the lake by over 350 feet and helping cut the Snake River. Afterward the lake formed the Provo Shoreline Terrace at just over 4,700 feet above sea level, then about 12,000 years ago its level fell sharply as the Great Basin climate changed, leaving the Gilbert Terrace as the last shoreline before the lake retreated into what is now the Great Salt Lake near Salt Lake City, Utah. The Bonneville Salt Flats, or Great Salt Lake Desert, remained after the lake evaporated, leaving a vast concentration of potassium, magnesium, lithium, and sodium chloride in deposits from less than one inch to six feet thick across 3,000 square miles. Among the flattest areas on earth, the flats are seasonally covered by shallow winter water that helps create their mirror-like, hard white surface, whose reflective salt produces coolness and shimmering mirages; the driest, purest salt forms an exceptional racing surface long used for speed and endurance records, including Ab Jenkins's 60 M.P.H. ride on a Yale Motorcycle in 1910 and Gary Gabelich's 622.40 M.P.H. run in the Blue Flame in 1970. The name Bonneville comes from Captain Benjamin E. Bonneville of the U.S. Army, in whose employ Joseph Reddeford Walker mapped the Great Salt Lake region and crossed the northern perimeter of the flats in 1833, though Bonneville himself likely never visited.