The ridge is primarily composed of the windblown glacially derived sediment known as loess. It contrasts greatly with the flat table land around it and with the black soil that makes up the delta. The ridge varies from half a mile to 12 miles (19 km) wide and reaches an elevation of 550 feet (170 m) near its northern extremity. The highest point on the ridge in Arkansas is "Legacy Mountain" at the Craighead County solid waste site south of Jonesboro. The formation is generally thought to have originally been an island between the Mississippi River and Ohio River that was isolated as a long low hilly ridge after the rivers changed course millions of years ago. Recent seismic evidence, however, questions the fluvial origin by focusing on uplift along ridge bounding faults.