unusual forms that occur in the Suisei Planitia. They are buried and rounded in profile, with only their rim crests rising above the surrounding smooth plains. Therefore, these craters cannot be assigned a specific age; they may be of any age from late c1 to late c3. Structure The most conspicuous structural elements in the quadrangle are the radial and concentric ridges and cracks inside the Caloris Basin and the ridges developed in the Odin Formation and smooth plains unit immediately outside Caloris. O’Donnell and Thomas (personal communication, 1979) have suggested, on the basis of orientation of features outside Caloris, that these ridges and scarps largely follow preexisting radial and concentric fracture patterns in the mercurian lithosphere initiated by the Caloris impact, similar in character to those around Imbrium on the Moon (Mason and others, 1976). Caloris itself consists of a single mountain ring and a weak outer scarp. A few sinuous scarps also occur in this quadrangle, including the Heemskerck Rupes which cuts the older intercrater plains. Scarps of this type are considered by Strom and others to be compressive thrust faults resulting from overall shortening of the mercurian crust early in its history. Geologic history The history of the Shakespeare quadrangle as evidenced by materials exposed at the surface begins with the formation of intercrater plains material and of impact craters both older and younger than these plains. Some c1 and c2 craters were superposed on the intercrater plains. The intermediate plains material and lineated plains unit were emplaced over the intercrater plains, as were most craters of c3 age. Then followed the major asteroidal impact that produced the Caloris Basin and the emplacement of rocks of the Caloris Group around the basin. Comparison of crater populations on surfaces older and younger than Caloris suggests that at the time of the Caloris impact, the population of craters smaller than 30 km in diameter was eradicated from the pre-Caloris terrain (Guest and Gault, 1976). Gault and others (1976) suggested that the smaller craters were destroyed by the Caloris event and by other basin-forming events elsewhere on the planet at about the same time. The smooth plains material was then emplaced. Some c3 craters were formed after the Caloris event and after some of the smooth plains were formed. Superposed on the smooth plains unit and on all older deposits were craters of c4 age, inside which was emplaced the