Mamers Valles is a long, winding canyon in the north of Mars. It covers 1000 km, cutting through the cratered uplands of the Arabia Terra, from the Cerulli Crater to the Deuteronilus Mensae near the edge of Mars' vast northern lowlands. Through its midsection, it averages a width of 25 km and a depth of 1200 meters. The most popular theory states that the canyon was likely formed by either water or lava, with the flow from south to north and additional material flowing from the slope toward the valley floor. (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.