Mawrth Vallis is a valley on Mars. It is an ancient water outflow channel with light-colored clay-rich rocks. Mawrth Vallis is one of the oldest valleys on Mars. It was formed in and subsequently covered by layered rocks, from beneath which it is now being exhumed. (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.