The Florida Reef (also known as the Great Florida Reef, Florida reefs, Florida Reef Tract and Florida Keys Reef Tract) is the only living coral barrier reef in the continental United States. It is the third largest coral barrier reef system in the world (after the Great Barrier Reef and Belize Barrier Reef). It lies a few miles seaward of the Florida Keys, is about 4 miles (6 to 7 km) wide and extends (along the 20 meter depth contour) from Fowey Rocks just east of Soldier Key to just south of the Marquesas Keys. The barrier reef tract forms a great arc, concentric with the Florida Keys, with the northern end, in Biscayne National Park, oriented north-south and the western end, south of the Marquesas Keys, oriented east-west. The rest of the reef outside Biscayne National Park lies within John Pennekamp Coral Reef State Park and the Florida Keys National Marine Sanctuary. Isolated coral patch reefs occur northward from Biscayne National Park as far as Stuart, in Martin County. Coral reefs are also found in Dry Tortugas National Park west of the Marquesas Keys. There are more than 6,000 individual reefs in the system. The reefs are 5,000 to 7,000 years old, having developed since sea levels rose following the Wisconsinan glaciation.