I was flying the DC-3 up from Little Rock toward Memphis, TN, at 5,000' MSL, when there began to appear in the distance a large, flat-topped land mass, something like the mesas of the American Southwest. It lies south of Memphis Intl. Airport. When I flew over the top of this "thing" rising uncharacteristically out of the otherwise flat terrain of that area, it looked like a high, flat plateau with 45-degree sides -- or a low mountain with the top sawed off. What the heck is THAT?