following excavation of very large multiring impact basins such as the one postulated under Borealis Planitia (Boyce and Grolier, 1977), were not unambiguously identified in the Borealis region. On one hand, some ridges on the surface of the smooth plains material in Borealis Planitia may be of structural (internal) origin; this type of ridge elsewhere on Mercury has been ascribed to compression and a slight shortening of the crust (Melosh, 1977; Melosh and Dzurisin, 1978). On the other hand, the wrinklelike sinuous ridge along the northeast border of the Goethe Basin, together with the outward-facing concentric scarps along its periphery, may represent the fronts of lava flows that are associated with the development of a structural moat between the basin fill and the wall. The latter interpretation supports the view that impact craters and basins on Mercury, as on the Moon (Schultz, 1977) and Mars, “have played a dominant role in controlling the surface expression of igneous activity” (Schultz and Glicken, 1979, p. 8033). Slow, long-lasting isostatic adjustment of the basin floor may have continued well after the emplacement of the basin fill, a structural situation similar to that of crater Posidonius on the Moon (Schaber and others, 1977, Schultz, 1977). In Borealis Planitia, however, most of the ridges are of external origin. They appear either to outline the rim crests of subjacent ghost craters that are lightly mantled by smooth plains material or to be lava flow fronts. The map shows the rim crests of 20 ghost craters, ranging in diameter from 40 to 160 km, that are buried under the smooth plains material of Borealis Planitia, which material is coextensive with the fill covering the floor of the Goethe Basin. In addition, ejecta from the crater Depréz extend more than 40 km eastward beyond a circular scarp that may represent the rim crest of a buried crater 170 km in diameter (FDS 156, 160) or, more likely, the front of lava flows. The size and density of these ghost craters suggest that, prior to emplacement of smooth plains material, the original heavily cratered surface of Borealis Planitia—which may have been the cratered floor of a very large multiring impact basin—and the cratered floor of the Goethe Basin were similar in composition and age to the intercrater plains material of the highlands to the west. Many scarps in Borealis Planitia are subconcentric to the rim of the Goethe Basin