Martian craters in the size range 10--250 km follow a log normal size frequency distribution law. Analysis techniques based on the log normal model yield possible evidence for the size-frequency evolution of crater-producing bodies. Some regions on Mars display excessive depletions of either large or small craters; the most likely cause of the depletions are considered. Apparently, eolian sedimentation has markedly altered the population of small craters south of -30¿ latitude. The general effects of crater obliteration in the southern hemisphere appear to be confined to diameters of less than 20 km. A strong depletion of large craters in a large region centered at 35¿ latitude and 10¿ west longitude may indicate locations of subsurface ice.