The rim of Posidonius is shallow and obscured, especially on the western edge, and the interior has been overlain by a lava flow in the past. The crater ramparts can still be observed to the south and east of the crater rim, and to a lesser degree to the north. There is a smaller, semi-circular rim of a concentric, flooded crater within the main rim, offset towards the eastern edge. There is no central peak, but the floor is hilly and laced with a
rille system named the
Rimae Posidonius. The floor is also slightly bulged due to the past lava uplift, which also likely produced the complex of rilles. The northeast rim is interrupted by the smaller crater Posidonius B. Within the crater rim, offset just to the west of center is another smaller crater Posidonius A. Posidonius is a crater of Upper (Late)
Imbrian age. On the Mare Serenitatis surface near Posidonius is a notable system of
wrinkle ridges that parallel the nearby shore. These are designated the
Dorsa Smirnov. At the peak of these ridges is a small crater, Posidonius Y, with a diameter of 2 km. This crater is surrounded by a patch of high-
albedo material, its
ray system. This peak was formerly designated Posidonius Gamma (γ). Posidonius Gamma was first observed by the lunar cartographer
Julius Schmidt in 1867, who noted the similarity to the bright patch surrounding the crater
Linné. ==Satellite craters==