Mangala Valles is a complex system of criss-crossing channels on Mars, located in the Tharsis region and in the Memnonia quadrangle. It originates in the Hesperian and Amazonian epochs. It is thought to be an outflow channel, carved by catastrophic floods, and the release of vast quantities of water across the Martian surface. This flooding was probably initiated by tectonic stretching and the formation of a graben, Mangala Fossa, at the channels' head, perhaps breaching a pressurized aquifer trapped beneath a thick "cryosphere" beneath the surface. (wikipedia entry)
A vallis is a valley. On Mars large valles are named after words for Mars/star in various languages. Small valles are named after classical or modern names of rivers.