The Tolstoj quadrangle in the equitorial region of Mercury contains the southern part of Caloris Planitia, which is the largest and best preserved basin seen by Mariner 10. This basin, about 1300 km in diameter, is surrounded by a discontinuous annulus of ejecta deposits of the Caloris Group (units cm, cn, co, cvl, and cvs) that are embayed and covered by broad expanses of smooth plains (unit ps). the southeast half of the quadrangle is dominated by ancient crater deposits (units c1, c2, and c3) by nondescript rolling to hummocky plains materials (units pi-ps). The ancient and degraded Tolstoj multiring basin, about 350km in diameter, is in the south-central part of the quadrangle. The large, well-preserved crater Mozart (285 km diameter) is a prominent feature in the western part of the area; its extensive ejecta blanket and secondary crater field are superposed on the smooth low plains (unit ps) surrounding Caloris.