Between the northern and southern portions of the Tharsis bulge lies a relatively narrow, northeast-trending region that may be considered Tharsis proper or central Tharsis. It is defined by the three massive Tharsis Montes volcanoes (Arsia Mons, Pavonis Mons, and Ascraeus Mons), a number of smaller volcanic edifices, and adjacent plains consisting of young (mid to late Amazonian) lava flows. The lava plains slope gently to the east where they overlap and embay the older (Hesperian-aged) terrain of Echus Chasma and western Tempe Terra. To the west, the lava plains slope toward a system of immense northwest-oriented valleys that are up to 200 km wide. These northwestern slope valleys (NSVs), which debouch into Amazonis Planitia, are separated by a parallel set of gigantic keel-shaped prominatories. The NSVs may be relicts from catastrophic floods of water, similar to the huge outflow channels that empty into Chryse Planitia, east of Tharsis. The area of central Tharsis is approximately 3500 km long and includes most of the region covered by the Tharsis quadrangle and the northwestern portion of the adjoining Phoenicis Lacus quadrangle to the south.