of the unit. In addition, the younger plains units exhibit densities of superposed craters ranging from moderate to very sparse. The intermediate plains material is older than the freshest large craters (100–150 km in diameter) but younger than all basins, and younger than all large craters that are more than moderately degraded. Thus, the material mapped as the intermediate plains unit overlaps in time of origin the tail end of the primordial bombardment. The stresses responsible for the elongate ridges and scarps must have occurred after the end of the primordial bombardment and after emplacement of the intermediate plains unit. Where smooth plains material abuts ridges and scarps, the evidence is mostly ambiguous because we cannot tell if ridge formation involved smooth plains material or if the ridges are upwarped intermediate plains material with smooth plains material ponded against them. On the floors of some craters, such as Gluck, scarps apparently offset material mapped as smooth plains, but the exposures are so small that this interpretation could easily be challenged. Ridges appear to be both older and younger than medium-size craters (30–60 km in diameter) on the intermediate plains unit, but intersections of ridges with craters in this size range are too rare to constrain the time of ridge formation. Thus, ridge formation obviously occurred after emplacement of the intermediate plains unit, but how long after remains uncertain in this quadrangle. Smooth plains material is apparently younger than all large craters, and hence is the youngest material in the quadrangle with the exception of the local material related to some very small craters (<20 km in diameter). Crater distributions Because of the varied and generally poor quality of the imagery, detailed cratering history cannot be inferred from these plots. However, three observations seem valid: the density of large craters is distinctly higher for intercrater than for intermediate plains material; for the intercrater and intermediate plains material, curves for craters with diameters between 3 and 15 km nearly coincide (the abundant, mostly degraded small craters characteristic of intercrater plains material but not characteristic of intermediate plains material are less than 3 km in diameter); craters of all sizes on the smooth plains unit are much less abundant than on the other units, although the smooth-plains plot is unreliable in detail because of the small total number of craters counted and the need to combine counts from isolated exposures. A serious sampling