post-Caloris events over a broad expanse of the surface of Mercury. Crater degradation chronologies, such as the one modified from Trask, and correlations between plains units on the basis of crater frequency may aid in tying much of the remainder of the surface of Mercury to the Caloris event. Unlike the Imbrium-related stratigraphy of Shoemaker and Hackman, that devised for Mercury is a rock rather than a time stratigraphy. It recognizes the existence of an orderly, in essence isochronous sequence of mappable units around Caloris that are similar in character to those recognized around the better preserved impact basins of the Moon such as Orientale, Imbrium, and Nectaris. Younger plains materials The Caloris floor plains material is a special problem and is not included in the Caloris Group. The plains have some features in common with the Maunder Formation in the floor of Orientale on the Moon but do not show the radial and circumferential ridges characteristic of the Maunder that led to its interpretation as a basin floor unit. The Caloris floor plains have a more open, coarser fracture pattern than does the Maunder. In addition, the Caloris ridges and the fractures cutting them have a crude rhombic pattern that led Strom and others to conclude that the plains materials subsided and then were gently uplifted to produce the open tension fractures observed. The ridges in the floor of Caloris lack the crenulated crests that are common on lunar ridges. Regardless of the origin and tectonic history of these plains, it seems clear that they represent a deep basin fill that obscures the original floor of the Caloris Basin. The largest single expanse of the smooth plains material surrounds the Caloris Basin—mostly in Tir and Budh Planitiae—but many smaller patches occur in crater floors and other topographic depressions within the heavily cratered terrain in the southeastern part of the quadrangle. The plains are characterized by a relatively sparse crater density and an abundance of mare-type wrinkle ridges; overlap relations indicate that the plains are younger than the more densely cratered units. The plains also embay the Caloris Formation and account in particular for the skeletal map pattern of the Van Eyck Formation. The ubiquitous distribution of smooth plains in topographically low regions supports the hypothesis that these materials were deposited in a fluid or semifluid state as basin ejecta or volcanic flows. The plains are thought to be slightly