The Rumker quadrangle, in the northwest quadrant of the Moon, is adjacent to the western rim of the multi-ring Imbrium basin and to Sinus Iridum, a large (220 km diameter) mare-filled crater. Both of these great depressions were probably formed by impact, as indicated here and elsewhere on the Moon by the characteristic form, distribution, and texture of surrounding materials and structures. The Imbrium basin and Iridum crater were filled by mare materials during the Imbrain and Eratosthenian Periods. In this quadrangle, the widespread ejecta blanket of the Imbrium basin, as well as the basin’s concentric ridges and mountain rings, has largely been buried by terra materials of mixed origin and by ejecta from Iridum and numerous smaller impact craters. These materials, together with some terra units of probable volcanic origin, make up the highland terrain. Mare materials of Oceanus Procellarum cover part of this westward-sloping highland shelf that separate it from the Rumker Hills, an isolated plateau surrounded by the mare. No visible discontinuity distinguishes mare material of Oceanus Procellarum from those in the Imbrium basin, both of which contain several units having similar albedo and color. Parts of the mare are very dark and smooth and appear relatively young, and parts of the terra contain many diverse landforms of Imbrian and younger age that appear to be volcanic. This region may therefore have been more active internally during late stages of lunar history than many others on the near side.