HiRISE images together with other recent orbital data from Mars define new characteristics of enigmatic Hesperian-aged deposits in Sirenum Fossae that are mostly 100-200 m thick, drape kilometers of relief, and often display generally low relief surfaces. New characteristics of the deposits, previously mapped as the "Electris deposits," include local detection of meter-scale beds that show truncating relationships, a generally light-toned nature, and a variably blocky, weakly indurated appearance. Boulders shed by erosion of the deposits are readily broken down and contribute little to talus. Thermal inertia values for the deposits are ̃200 J m -2 K -1 s -1/2 and they may incorporate hydrated minerals derived from weathering of basalt. The deposits do not contain anomalous amounts of water or water ice. Deflation may dominate degradation of the deposits over time and points to an inventory of fine-grained sediment. Together with constraints imposed by the regional setting on formation processes, these newly resolved characteristics are most consistent with an eolian origin as a loess-like deposit comprised of redistributed and somewhat altered volcanic ash. Constituent sediments may be derived from airfall ash deposits in the Tharsis region. An origin directly related to airfall ash or similar volcanic materials is less probable and emplacement by alluvial/fluvial, impact, lacustrine, or relict polar processes is even less likely.