Rille

Hadley Rille on the southeast edge of the Moon’s Mare Imbrium. Modified from Source: Apollo Image AS15-M-1134

Long narrow depression on the surface of the Moon; also called “sinuous rilles”. Lunar rilles usually flow away from small pit structures and probably mark lava channels or collapsed lava tubes that formed during mare volcanism. In some cases, the lunar flows may have melted their way downwards into older rocks. Hadley Rille, visited by the Apollo 15 astronauts, is shown here.