This large martian outflow channel, Ma'adim Vallis, is about 20 kilometers wide and 2 kilometers deep in this area and at least 700 kilometers long overall. Ma’adim Vallis formed in the ancient martian highlands and may be as old as 3.5 billion years. Similar outflow channels drain into the Chryse Basin on Mars. Channels such as these are among the best evidence for abundant quantities of groundwater or ice in the early history of Mars. They may have formed by the sudden release of massive amounts of groundwater or melted ground ice. Channel enlargement has been due to erosion and slumping of the rim. Some of the short narrow channels along the walls of Ma'adim are probably sapping channels. [Numerous side canyons at Valles Marineris (slide #24) and the Grand Canyon (slide #12) may also be sapping channels.] Sapping occurs when groundwater partially dissolves and undermines the rock, which collapses into debris deposits and is carried away by other erosional processes.
1 images 597A56, 631A58.