Material exposed on the surface of the moon is heterogenous. The albedo and other physical characteristics that have been determined with the use of optical and radio telescopes vary from one part of the moon from another, and the variations are partially correlated with differences in topography. Discontinuities in the areal variation permit the surface material to be divided up into map units, each exhibiting a limited range of topographic characteristics. Each map unit is further characterized by a distinctive pattern of distribution, and the patterns of certain units are in places superimposed on the patterns of other units. From the relations of superposition it is possible to determine the sequence in which the units were formed.
Geologic map of the Aristarchus region of the Moon