Mars Orbiter Laser Altimeter (MOLA) topographic data support the presence of an extensive Hesperian-aged volatile-rich south polar deposit, the Dorsa Argentea Formation (DAF) and related deposits, underlying the present Amazonian-aged cap. The eastern margin of these deposits displays further evidence for meltback, ponding, and drainage of the volatile-rich deposit. Channels leading from the margins of the DAF deposit enter nearby craters and are interpreted to represent drainage of water, ice, and sediment from the DAF. Channels connecting these craters provide evidence for extensive crater flooding, ponding (minimum volumes ~1012 m3), overtopping, downcutting, and further drainage of material through a series of craters and into the Prometheus Basin near the edge of the current cap. Topography data show that water filled some craters to depths of at least 200 m and possibly as much as 600 m. Materials derived from melting of this lobe of the DAF drained over a lateral distance of ~600 km and a vertical height of ~800 m, partially emptying into the Prometheus Basin. These combined observations imply that meltback of the older, volatile-rich deposit was a widespread event, occurring on both sides of the deposit over a distance of at least 1800 km, and in an area possibly as much as ~2.9 ¿ 106 km2. Candidate top-down and bottom-up melting scenarios are examined; the presence of associated Hesperian-aged volcanic deposits and possible subglacial edifices suggests that local and regional basal heating was a major factor in generating the meltwater that formed the drainage features.