The surface of the Moon is heterogeneous both on a regional and a fine scale. The geologic mapping of this surface is similar in principle to the geologic mapping of the Earth's surface and depends on the discrimination of similarities and differences between materials and features at different points on the surface. Differences from area to area in characteristic topographic forms and in such physical properties as albedo indicate that the areas are underlain by materials of different nature and have formed under different processes. The units shown on the map are analogous to stratigraphic units adopted for mapping terrestrial geology in that they are bodies of rock with a limited range of properties and age and are of relatively simple geometric form. It is inferred that the material of the individual rock units mapped has formed under a limited set of conditions. The relative ages of some of the map units can be determined where they overlap. For example, the rays around Copernicus overlap the craters Eratosthenes and Reinhold and the rim materials of Copernicus, Eratosthenes, and Reinhold overlap the smooth materials of the maria.
