The surface of the Moon is heterogeneous. Surface materials are classed on the basis of telescopic observations into units, each having a limited range of topographic and other physical such as albedo, the reflectivity under full-Moon illumination. Such units are considered analogous to the rock-stratigraphic formations of terrestrial geology. By application of the principals of superposition and intersection, these rock units 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 subdivision series; corresponding to these units are periods and epochs of time, respectively (Shoemaker, 1962,; Shoemaker and Hackman, 1962). The type localities of most the systems and series recognized to date are in the vicinity of Mare Imbrium. Correlation of many of the rock-stratigraphic units of the Hevelius region with the rock-stratigraphic units in the Mare Imbrium region is still uncertain.
