Grand Gulf State Park near Thayer, Missouri, consists of 322 acres (130 ha) encompassing a forked canyon that is the remnant of an ancient collapsed dolomite cave system. An uncollapsed part of the original cavern roof spans 200 feet (61 m), creating the largest natural bridge in Missouri. It was declared a National Natural Landmark in June 1971 for its excellent example of karst topography and underground stream piracy.
The Grand Gulf is nearly 1 mile (1.6 km) long and up to 130 feet (40 m) deep with sheer sides. A watershed of 26 square miles (67 km²) feeds into the gulf which itself drains into a cave entrance at its eastern end. Dye traces have shown that water entering the cave in Grand Gulf emerges 1 to 4 days later at Mammoth Spring in Arkansas, 9 miles (14 km) distant.
