Posidonius (56 miles) is an older formation with rather low walls. There is a nice 6 mile crater off center on the floor and near a small central peak. There is a long ridge, beginning at the southeast and more or less paralleling the eastern rim, curving into the northern floor. In my 6 inch refractor there are several rills visible. The most prominent is the one near the western wall and ends at a ridge. There is a lot of detail on the floor of this old formation. Examine it 5 days after new Moon.
Chacornac (31 miles) is below Posidonius with walls less than a mile high. The floor is rough and there is a little 3 mile crater near the center.