Wargentin is an unusual lunar impact crater which has been filled to its rim by a basaltic lava flow, forming a raised plateau. When the lava flow occurred, it erupted from within the crater walls and proceeded to accumulate until overrunning the lowest portion of the rim. Some blockage then prevented the lava flow from returning to equilibrium. Since the time when this occurred, some ejecta has been deposited across the top, giving the surface a higher albedo than is typical for deposits of basalt. (wikipedia entry)