Using a new photoclinometric technique with high-resolution Mariner 9 images, maximum slopes of 10¿--20¿ are found to occur on an exposure of layered deposits within the south polar residual cap of Mars. Stereophotogrammetry is used to constrain the photoclinometric solutions, which resolve layer thicknesses of 100--300 m. Albedo variations are correlated with slope, indicating that frost is present on level areas. There is evidence for temporal changes in frost distribution in the 7 days (4¿ of Ls) between the two images used in this study. The magnitude of the slopes derived here and consideration of the stability of water ice at the surface of the layered deposits strongly suggest the presence of a competent weathering rind. The weathered surface may be composed of dark filamentary sublimation residue particles that protect the underlying ice from solar heating. This hypothesis is consistent with previous studies of the regional color and albedo of the layered deposits, which indicate that the deposits are slightly darker and less red than the bright dust that mantles much of the south polar region. Furthermore, the proposed weathering mechanism provides a plausible source of dark, saltating material for the Martian polar dune fields. ¿ American Geophysical Union 1990