The surface of the Moon is heterogeneous. Surface materials are classed on the basis of telescopic observations into map units, each having lateral continuity and a limited range of physiographic characteristics and optical properties (mainly polarization and albedo, the reflectivity under full moon illumination). Such units are analogous to the rock-stratigraphic formations of terrestrial geology. By application of the principles of superposition and intersection, these formations are arranged in order of relative age and grouped into time-stratigraphic units. Following terrestrial convention, the major time-stratigraphic units are designated systems, and their subdivisions series; corresponding to these are periods and epochs of time (Shoemaker, 1962; Shoemaker and Hackman, 1962).
(LAC-42) Geologic map of the Mare Serenitatis region of the moon