Posidonius Rille
This image shows a sinuous rille located in the crater Posidonius. The
source of this rille is a shallow pit on the northern crater wall (arrow).
The rille then meanders along the edge of the crater floor for over 60
miles (100 km). Finally, it reaches a hole in the
crater rim and vanishes into Mare Serenitatis. Note that the rille is
also only the last stage of mare volcanism within this crater. It lies on
top of an older, smooth mare unit which buries part of the crater floor
and much of the western crater wall. ( The black bar in the lower right is
about 6 miles (~10 km) long.) (Lunar Orbiter image IV-86-H3)