and older plains units are rare. Smooth plains material thus is defined almost entirely by texture and apparent crater density. Few superposition data directly support the inferred age sequence, but the relative youth of the smooth plains unit is indicated by its presence on the floors of craters that are superposed on intermediate plains material. The smooth plains unit probably includes materials of a wide range in age, but the exposed areas are too small to test this possibility quantitatively. Although a volcanic origin cannot be ruled out for all or part of the smooth plains material, it is more probably a mixture of ejecta from very small craters and colluvium mass wasted from crater walls. Structure The ridges associated with the intermediate plains unit are best interpreted as tectonic in origin because they extend into adjacent exposures of intercrater plains material and, more significantly, because they transect ejecta, rims, and floors of craters. The ridges range in length from about 50 km to many hundreds of kilometers, are sinuous to lobate in plan, and generally trend about north-south. Most are asymmetric, with one slope steeper than the other, and at places they can be more logically referred to as rounded scarps. Commonly, an individual ridge changes along trend from symmetric ridge to asymmetric ridge to rounded scarp. Strom and others interpreted most of these features to be surface expressions of thrust faults, and we can find no evidence within the Victoria quadrangle not already considered in their discussion. Because of their globally systematic orientations, these ridges and scarps have been associated with stresses developed by tidal despinning of Mercury. However, most trend approximately north-south and thus do not fit the pattern expected in the midlatitude belt, unless stresses from overall contraction were superposed on the stresses due to despinning. Geologic history The oldest material and features in the Victoria quadrangle are the intercrater plains material and areally associated, severely degraded basins. No craters are clearly older than intercrater plains material, and the relative ages of the cl basins are ambiguous. Numerous large craters are superposed on intercrater plains material; by analogy with lunar and martian history these craters most likely date from more than about 4 b.y. ago. The available evidence suggests a relatively long history of plains formation. Some of the material included in the intercrater plains unit appears to have been plainslike before the intense cratering characteristic