The area included in the Sinus Sabaeus quadrangle represents an ancient cratered surface that has been modified by a combination of impact cratering, fluvial, eolian, and, probably, volcanic processes. Impact cratering has been the dominant process. Cumulative frequencies of craters larger than 20 km for the entire quadrangle (fig. 4) are only about a factor of two below the idealized lunar steady-state curve (Moore, 1964; Trask, 1966). Two of the units mapped (hilly, channeled, and cratered material and hilly and cratered material) have cumulative frequencies of craters larger than 40 km that are near the same as the idealized steady-state curve. The young map units (smooth plains material and units of the crater Bakhuysen) have cumulative frequencies of craters a distributions that are similar in form and magnitude to the average frequency distributions of lunar maria. Ages of surfaces recorded by craters larger than 5 km (fig. 4) are probably measured in several billions of years. Direct comparison with lunar craters would place the age of the young units (smooth plains material and units of Bakhuysen) in the range 3.2 to 3.5 billion years, but procedures for making such a comparison are controversial. (See, for example, Soderblom and others, 1974; Neukum and Wise, 1976). These units may be either a few tenths of a billion years older or about a billion years younger than the average lunar maria. The more cratered units (hilly, channeled, and cratered material and hilly and cratered material) may represent the cumulative result of about 4 billion years of cratering (Soderblom and others, 1974; Neukem and Wise, 1976).