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 physical properties such as topography, visible under low-illumination and albedo, the reflectivity under full-Moon illumination. Such units are considered analogous to the rock-stratigraphic formations of terrestrial geology. By application of the principles of superposition and intersection, these rock units are arranged in order of relative age and grouped into time stratigraphic units. Following terrestrial contention, the major time-stratigraphic units are designated systems, and their subdivisions, series; corresponding to these units are periods and epochs of time, respectively (Shoemaker, 1962;Shoemaker and Hackman, 1962). The type localities of most of the systems and series recognized to date are in the vicinity of Mare Imbrium. The assignment of the various mappable rock-stratigraphic units to time-stratigraphic units is somewhat uncertain except in the type areas.
(LAC-60) Geologic map of the Julius Caesar quadrangle of the moon