Valles Marineris is a canyon that was discovered south of the Martian equator.  The canyon is very long.  It extends the distance from coast to coast on the U.S. with a length of 4000 km long.  Smaller canyons, 50 to 100 km wide combined into a larger canyon 600 km wide.  The deepest part of Valles Marineris reached a depth of 10 km.  Scientists think this canyon was made first by the surface collapsing then, later, layered material filled the canyon. Soon the faults widened and possibly water spilled out and rushed towards nearby channels.  Finally, the wind created dunes on the floor of Valles Marineris.

Information from: http://astrogeology.usgs.gov/Projects/VallesMarineris/