Mark Twain Memorial Bridge (1936) Mark Twain Memorial Bridge (1936) The Mark Twain Memorial Bridge was a Cantilevered Through Truss Bridge carrying US 36 over the Mississippi River. It was replaced in 2000 with the extension of I-72 and subsequently demolished. The bridge, which replaced motor vehicle use of the Wabash Bridge, was opened in 1936 by President Franklin D. Roosevelt and entered Missouri at the foot of Cardiff Hill which appeared in Twain's books The Adventures of Tom Sawyer and The Adventures of Huckleberry Finn. A crosswalk crossed the highway, connecting the Twain boyhood home with the lighthouse at the top of the hill.
Mark Twain Cave and stalactites in large open areas. Mark Twain Cave and Cameron Cave instead have a multitude of narrow, winding passages. The caves are made mostly of a soft limestone called Louisiana Lithographic Limestone and found only in a area around Hannibal and Louisiana, Missouri. The limestone has been estimated by geologists as around 350 million years old, while the cave passages were formed some 100 million years ago. Mark Twain Cave covers some consisting of four entrances and 260 passages, maintaining a year around temperature of . While it is possible that prehistoric Native Americans were aware of the cave,