Scars and deposits from landslides, catastrophic movements of rock and soil, are common along the walls of the Valles Marineris. Typically, rock breaks away from the wall along a curved fault or slip surface, and first slides downhill as fairly solid slump blocks. A portion of the slip face is left exposed as the landslide scarp or headwall. As rock and soil slide downhill, the "solid" slump blocks break apart and the resulting outflow of rock and soil, mixed with air and/or water, can move like a liquid. In the Valles Marineris, these outflow "liquids" flowed faster than 100 km/sec. As this part of the landslide flows, it spreads out to give a thin, extensive debris apron. Landslides like these are also common in impact craters, where the steep walls of the craters can fail, and slump blocks and debris aprons slide into the craters' centers.