rille

views updated May 08 2018

rille (rima) A small valley on the Moon. Three types are recognized. (a) Straight rilles are typically 1–5 km wide and hundreds of kilometres long, unrelated to surficial topography, analogous to terrestrial fault grabens. (b) Curved or arcuate rilles are variants of straight rilles, with similar dimensions, and form concentrically to major ringed basins (e.g. Mare Humorum). (c) Sinuous and meandering rilles, formed by thermal erosion by flowing lava. Hadley Rille, 1.2 km wide, 270 m deep, and 135 km long, visited by Apollo 15, is the type example.