back of card: The Bonneville Salt Flats is a salt deposit left by the receding of ancient Lake Bonneville. This deposit covers about 159 square miles extending some nine miles along U. S. Highways 40 and 50 and the Western Pacific Railroad. The salt is white, crystalline aggregate, porous, hard and rigid so that it supports loaded trucks. Aug 26. 1939, John Cobb again became the world's automobile speed king by driving his car 369.74 miles per hour.