surface viewed by Mariner 10. The principal morphologic characteristic of the intercrater plains material is the high density of superposed craters 5 to 10 km in diameter, which are commonly shallow and elongate; probably they are secondary craters derived from nearby large primary craters that are superposed on the unit. As one group, the large craters and associated intercrater plains form some of the heavily cratered terrain defined by Trask and Guest. The relative age and nature of intercrater plains material are as uncertain in the Borealis region as they are elsewhere on Mercury. Strom noted the similarity in surficial morphologies between mercurian intercrater plains and pre-Imbrian pitted plains south-southwest of Mare Nectaris on the Moon (Wilhelms and McCauley, 1971; Scott, 1972). The pits in the lunar pre-Imbrian pitted plains are similar to the small secondaries that pepper the surface of Mercurian intercrater plains material. On the Moon, pre-Imbrian pitted plains material embays the Janssen Formation (Scott, 1972), whose base is defined as the base of the Nectarian System (Stuart-Alexander and Wilhelms, 1975). However, the crater density of the intercrater plains material in the Borealis region matches that of an area on the far side of the Moon, in the region northwest of crater Tsiolkovskiy bounded by crater Mendeleev and Mare Smithii. This area is dominated by pre-Nectarian unmantled terra and pre-Nectarian and Nectarian craters (Wilhelms and El- Baz, 1977). The similarity in crater density of intercrater plains material on Mercury and of pre-Nectarian terrain on the Moon is geologically significant, inasmuch as it shows that the oldest recognizable surfaces on both Mercury and the Moon went through similar stages of crustal cratering, but not necessarily at the same absolute geologic time. Differences in crater density as well as embayment relations in the Borealis region show that the intercrater plains material and the smoother intermediate plains material are younger than many craters in the area northeast of crater Turgenev, and older than smooth plains material in Borealis Planitia. The relative age of intercrater plains material has a bearing on its origin. If very old, intercrater plains material may consist of anorthosite derived from a magma ocean such as may have existed on the Moon (Wood and others, 1970). If emplaced during later stages of mercurian evolution, it may consist of basin ejecta or lava flows. However, planetwide, the morphologic evidence for an impact origin rather than a volcanic one is