The great November 5, 1985 Potomac Valley flood was responsible for the release of 1800 m3 of alluvial and colluvial sediment from the walls of the entrance doline of Mystic Cave. Flood waters were sufficiently powerful to flush the entire mass of sediment not only into the cave but through the cave. Remnants of the sediment mass in the form of sand bars and a few cobbles wedged in speleothems were the only evidence in the cave that the huge mass of sediment had moved through. The sediment moved as a suspended mass in water moving at peak velocities of many meters per second. Present day cave sediments must be interpreted with the understanding that entire sediment fillings can be transported or rearranged by single extreme events.