fills a much degraded, unmapped, irregular crater at 69° N., 157° W. (FDS 088). These knobs are about northeast of Caloris Montes and may represent Caloris Basin ejecta. Alternatively, they may be associated with crater Verdi ejecta or with lineated and secondary-crater ejecta that flare southeastward from an unnamed crater north of and adjacent to the crater Nizami. Another morphologic feature that may be related to the Caloris Basin event consists of grooves on intercrater plains material and on the southwest-facing walls of craters such as Mansart. These grooves are as much as several kilometers long and several hundred meters wide. The direction of elongation of many small secondary craters also suggests an origin related to the Caloris event. Younger plains material Smooth plains material (unit ps) forms the vast expanses of Borealis and Suisei Planitiae, as well as most basin and crater floors. It is the most extensive stratigraphic unit in the Borealis region, covering 30 percent of the mapped area. The surface of the smooth plains material is rather sparsely cratered compared to that of the intercrater plains material. Wrinkle ridges are common. Both the floor of the Goethe Basin and the younger craters (now observed as buried craters) superposed on it are mantled by smooth plains material; the unit also fills ghost and flooded craters that are common on both Borealis and Suisei Planitiae and resemble the lunar crater Archimedes. The enormous volume of smooth plains material that must underlie Borealis Planitia in order to bury pre-existing topography, as well as the presence of the material in basin and crater floors, suggest that the smooth plains material was emplaced in a fluidized state as volcanic lava flows (Murray and others, 1974). Even though flow fronts cannot be unambiguously mapped on Borealis Planitia, further evidence of the unit's volcanic origin is supplied by its overlap onto intercrater plains material, best observed along the west edge of Borealis Planitia (FDS 85, 152, 153, 156, and 160). The various types of plains material recognized on Mercury exhibit little tonal contrast. The albedo of smooth plains material is higher than that of lunar mare material (Hapke and others, 1975). The similarity in albedo between mercurian smooth plains material and lunar light plains material led Wilhelms to extend the analogy to composition: he suggested that both units consist of impact ejecta similar to the lunar Cayley Formation sampled by Apollo 16. Wilhelms