(LAC-78) Geologic map of the Theophilus quadrangle of the moon

The surface of the moon is heterogeneous. The differences from area to area of characteristic topographic forms and of physical properties such as albedo (reflectivity under full-Moon illumination) indicate that the materials underlying the surface vary from one area to another and that different processes have operated in the development of the surface. In lunar geologic mapping, areas are delineated that, as far as possible, are analogous in terrestrial mapping to the areas of outcrop of stratigraphic units - bodies of rock whose limited range of lithology and age and relatively simple geometric form indicate formation under a common set of conditions. For the moon, at the present time, physical properties that are believed dependent on lithology rather than lithology itself must characterize units. Age relations between units can be established from their boundary relations if certain simple assumptions are made about processes on the Moon - i'or example, that most exposed units were deposited at the surface. A sequence of time-stratigraphic units based on events that affected much of the surface of the Moon has been established in the vicinity of Mare Imbrium, and may serve as a general framework for lunar stratigraphy. The systems and salient events of each are as follows (Shoemaker, 1962; Shoemaker and Hackman, 1962).