A new bridge was built just downstream from here in 1931, with a grand opening scheduled for July 4 to coincide with the Killdeer Mountain Roundup Rodeo, but torrential rain forced postponement and many people left before the festivities were held the next day in a sea of mud. Bad luck continued when the roads leading to the bridge, which were supposed to be paved, went unbuilt because the onset of the Great Depression left no money available for construction. Locals dubbed the modern highway bridge, with only dirt roads leading to it, "Lost Bridge," and the name stuck. The approach roads were graded and graveled in 1953, then paved in 1968, and the original structure was replaced in the 1990s. Deposits of lignite coal here formed from vegetation that grew in swamps fifty to sixty million years ago, scoria formed when a layer of coal burned and baked the clay above it, grayish canyon layers formed from volcanic ash, and cream-colored layers are sandstone, siltstone, and other rocks formed in bodies of water.