1980). Surrounding Caloris is a discontinuous annulus of its ejecta deposits, called the Caloris Group. Caloris ejecta are embayed and partly covered by a plains unit that lies mostly in large, roughly circular depressions, some of which may be ancient degraded basins. This plains material also occurs in the floors of old craters and in small irregular topographic lows. The eastern part of the Shakespeare quadrangle consists mainly of cratered terrain and intercrater plains. Over the whole of the mapped area are scattered fresh craters superposed on other units; in the eastern part the large fresh craters show well- developed bright rays. Stratigraphy Pre-Caloris materials The oldest recognizable unit in the quadrangle is the intercrater plains material. These plains were originally described by Trask and Guest as intercrater plains. The unit has a surface expression of rolling to hummocky plains in the areas between large craters and is exposed mainly in the eastern part of the mapped area. The surface of the unit is pockmarked with craters, many of which are small (about 5 to 10 km in diameter), elliptical, and shallow; they are inferred from their shape to be secondary craters associated with larger craters and basins. Trask and Guest concluded that the surface of these plains represents a primordial surface of Mercury on which craters have been superposed. The large extent of this surface compared to its counterpart on the Moon was thought to reflect the restricted distribution of ejecta around each individual crater caused by the relatively high gravity on Mercury. Because of this high gravity, considerable areas were unaffected by crater and basin ejecta. However, Malin and Guest and O’Donnell (1977) have shown that in some areas the intercrater plains overlie highly degraded craters, a relation suggesting either that the intercrater plains were formed during a specific time in Mercury’s history and that cratering occurred both before and after their emplacement, or, alternatively, that the intercrater plains were formed by a continuous process throughout cratering history. In several parts of the quadrangle, especially on the margins of large expanses of smooth plains materials, is a unit of smoother and less rolling plains that have a lower crater density. Following Schaber and McCauley (1980), this unit is called intermediate plains material. It is difficult to map with precision because it grades into both the intercrater plains and the smooth plains. Also, its recognition depends on lighting conditions