to hummocky plains that lie between large craters in the southeastern part of the quadrangle make up the oldest recognizable map unit, the intercrater plains material. The plains were originally described as intercrater by Trask and Guest, who noted their level to gently rolling appearance and their general lack of well-defined craters larger than about 50 km in diameter. Malin showed the plains to contain highly eroded remnants of large craters and basins that are only very shallow circular depressions. These intercrater plains are, however, marked by a very high density of superposed craters that are small (5–10 km diameter), elongate, shallow, and probably secondary to the many large craters superposed on the plains. Superposition of crater ejecta over parts of intercrater plains in other areas indicates that some large craters formed in a preexisting intercrater plains unit. On the other hand, the intercrater plains material partly postdates some of the major cratering events on Mercury, according to apparent superposition relations. In particular, the unit appears to overlap the entire northwest side of the Tolstoj Basin, a feature indicating that the intercrater plains in this region probably do not represent the remains of the primordial surface of the planet. A complex history of contemporaneous craters and plains formation is therefore suggested. A detailed discussion of the origin of the intercrater plains on the Moon and Mercury was given by Strom. Patches of less cratered, smoother, less rolling plains occur throughout the quadrangle, but their recognition is highly dependent on the resolution and lighting of individual Mariner 10 frames. Therefore, because their distribution cannot now be mapped accurately, many of these patches are included with the smooth plains material. Certain patches of these intermediate plains, where clearly rougher and possibly older, are mapped as the intermediate plains material. These patches occur mostly within the floors of ancient craters and are distinguished by a slightly greater density of small craters and a lower incidence of small bright-halo craters than are found on the smooth plains material. The presence of plains intermediate in roughness and crater density between the oldest plains and the post-Caloris plains suggests that plains formation was a more or less continuous process that spanned much of the early geologic history of Mercury. Basin materials The impact that produced the Tolstoj Basin occurred very early in the history of the quadrangle. Two ragged, discontinuous rings approximately 356 km and 510