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 Hot molten or semifluid rock derived from a volcano or surface fissure from a differentiated and magmatically active parent body. Click on Term to Read More 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.