RILLE

(German: furrow), any of various valleys or trenches observed on the surface of the Moon. The term was introduced by early telescopic observersprobably Johann Schrter about 1800to denote such lunar features. Rilles measure about one to five kilometres wide and as much as several hundred kilometres long. They are often classified into two main types: straight rilles and sinuous rilles. Those of the first variety are flat-floored and relatively straight; they are occasionally associated with crater chains and sometimes arranged in an echelon pattern. These structures are thought to be grabens formed by floor collapse between parallel faults. The sinuous rilles, on the other hand, resemble winding river valleys. They may be similar to flow channels created by lava flows on the Earth. Apollo 15 astronauts visited the sinuous Hadley Rille and found a V-shaped valley that was apparently filled with rubble from lava-stratified walls; their observations, however, did not clarify the feature's origin.