High spatial resolution (1.5 to 6 m/pixel) Mars Global Surveyor Mars Orbiter Camera images obtained September 1997 to June 2001 show that each of the large, dark wind streaks of western Arabia Terra originate at a barchan dune field on a crater floor. The streaks consist of a relatively thin (<1 m) coating of sediment deflated from the dune fields and their vicinity. In most cases, this sediment drapes over a previous mantle that more thickly covers nearly all of western Arabia Terra. No concurrent eolian bedforms are found within the dark streaks, nor do any dunes climb up crater walls to deliver sand via saltation to the streaks. The relations between dunes, wind streak, and subjacent terrain imply that dark-toned grains finer than those that comprise the dunes are lifted into suspension and carried out of the craters to be deposited on the adjacent terrain. Previous eolian physics and thermal inertia studies suggest that, under modern Martian conditions, such grains likely include silt (3.9--62.5 ¿m), very fine sand (62.5--125 ¿m), and possibly fine sand (to ~210 ¿m). The streaks change in terms of extent, relative albedo, and surface pattern over periods measured in years; however, through June 2001, very little evidence for recent eolian activity (dust plumes, storms, dune movement) was observed.