a series of major impacts that have emplaced ejecta over widespread areas; the rock units associated with these impact basins were used to divide the lunar stratigraphic column into a series of well-defined time units (Shoemaker and Hackman, 1962; McCauley, 1967; Wilhelms, 1972). These relations are particularly clear for the Imbrium Basin (Wilhelms and McCauley, 1971) and Orientale Basin (Scott and others, 1977). Recognizable ejecta units extend outward from the Caloris Basin as far as one basin diameter; these units can be used to divide the mercurian stratigraphic column in much the same way as basin ejecta were used on the Moon. A stratigraphic and structural comparison between the Caloris and Orientale Basins has been made by McCauley (1977). In the Shakespeare quadrangle, only a lineated facies of the Van Eyck Formation is recognized, whereas in the Tolstoj quadrangle to the south, both it and a secondary crater facies are mapped (Schaber and McCauley, 1980; McCauley and others, 1981). Post-Caloris materials The plains material that forms the floor of the Caloris Basin has not been included in the Caloris Group and is mapped separately from the smooth plains. In many way, the Caloris-floor plains are similar to the smooth plains, except that they have been buckled and fractured into numerous ridges and grooves that intersect to form a grossly polygonal pattern. The dominant trends of these features are concentric and radial to the center of Caloris. On the basis of photometric evidence, Hapke and others (1975) suggested that the central part of the basin floor may be 7±3 km lower than the outer edge. Strom and others argued that the ridges were formed by compressive stress generated by subsidence of the floor, and the fractures by subsequent uplift of the center of the basin to produce crustal lengthening and the observed fracture pattern. The origin of the material itself is doubtful. It may consist of sheets of volcanic material emplaced shortly after the basin was formed or it may be material formed by the Caloris event either as melt or as the upper part of a plug of plastic material that rose in the crater floor as part of the impact process. Whatever the origin of this material, it seems clear that it covers the original floor of the excavated crater. The smooth plains material forms essentially level tracts, flooring depressions in the mercurian surface. The most extensive of such