New slope measurements from the Mars Orbiter Laser Altimeter (MOLA), in conjunction with the Rock Mass Rating system (RMR), permit inversions of slope height and angle from wallrock and interior deposits within Valles Marineris troughs for strength and lithology. Wallrock (50 < RMR < 65) is stronger than interior deposits (30 < RMR < 55). These values are consistent with layered igneous rock beneath wallrock slopes, with interior deposits consistent with partially indurated sedimentary or volcaniclastic rocks. Larger volumes of landslide debris relative to terrestrial slides are related to the reduced Martian gravity. Seismicity along trough-bounding normal faults likely triggered the landslides in the adjacent wallrock. The weaker interior deposits are located sufficiently far from a border fault (and on its hangingwall) to have remained stable during the seismogenic faulting in Valles Marineris.