The cratered terrain, a rolling surface covered by numerous impact craters and centered on Ariel's south pole, is the moon's oldest and most geographically extensive geological unit. It is intersected by a network of scarps, canyons (graben), and narrow ridges mainly occurring in Ariel's mid-southern latitudes. The canyons, known as "chasmata", probably represent graben formed by extensional faulting, which resulted from global tensional stresses caused by the freezing of water (or aqueous ammonia) in the moon's interior (see below). They are 15–50 km wide and trend mainly in an east- or northeasterly direction. The floors of many canyons are convex; rising up by 1–2 km. Sometimes the floors are separated from the walls of canyons by grooves (troughs) about 1 km wide. The widest graben have grooves running along the crests of their convex floors, which are called "valles". The longest canyon is Kachina Chasma, at over 620 km in length (the feature extends into the hemisphere of Ariel that "Voyager 2" did not see illuminated).