Once a crater has been formed, it will degrade over time as gravity and isostatic rebound modify the crater walls and floor. The central portion of a large crater will rise by isostatic uplift that results from removal of overburden. Walls may collapse by slumping to produce terraced crater walls that form the rings in multi- ring basins. Continued bombardment causes the crater to become less and less distinct from its surroundings. Degradation also can be attributed to lava flows, atmospheric processes such as weathering and erosion, groundwater processes (sapping), and tectonism. Thus old craters may exhibit only remnants of the original crater walls.