that vary across the mapped areas, especially east of longitude 120°. The presence of this unit suggests that the plains-forming process spanned much of the early geologic history of Mercury and continued long after the peak of cratering. In the southern part of Sobkou Planitia, intermediate plains have a lower albedo than the adjacent plains. In some places, they may simply represent areas of intercrater plains that have been partly flooded by the younger smooth plains material. Lineated plains material was recognized by Trask and Guest as forming terrain consisting of lines of hills and valleys, some of which are as much as 300 km long. This unit modified older large craters and intercrater plains. Its features are similar to those of the lunar Imbrium sculpture (Gilbert 1893) and to the hills and valleys radial to the Nectaris Basin on the Moon (Stuart-Alexander, 1971). The lineations were probably formed in a similar way to those of the Imbrium sculpture, which resulted from excavation by projectiles ejected at low angles from the Imbrium Basin; however, some of the mercurian valleys may be the result of faulting. Most of the lineated material in the Shakespeare quadrangle appears to be subradial to an ancient basin lying between Odin Planitia and Budh Planitia centered at latitude 28° N., longitude 158° W. However, except for its northernmost exposure, the surface of this unit is mantled by a facies of the Odin Formation. Hilly plains material consists of low, rounded, closely spaced hills with relatively few superposed craters. The hills range in size from 1 to 2 km across and were estimated to have heights of 100 to 200 m by Trask and Guest, who first recognized this unit and named it hilly terrain. The main tracts of hilly material occur in a roughly concentric band outside the Caloris ejecta. It is possible that this unit is associated with Caloris, although apart from geographical distribution, there is no supporting evidence. In some places, contact relations suggest that the hilly plains material may be older than intermediate plains material. Also, patches of the hilly material may be associated with intercrater plains materials in the eastern part of the quadrangle, where lighting conditions do not allow its recognition. Caloris group Rock units associated with the Caloris Basin are particularly important for the stratigraphy of Mercury. It has been demonstrated that the history of the Moon was punctuated by