This observation shows the contact between the hematite bearing plains and etched terrain in northern Meridiani Planum.
The hematite bearing plains ( exposed at the bottom left of the full image ) are dark, smooth and full of dune fields. This unit is laterally extensive and the same unit that the Mars Exploration Rover Opportunity was sitting on about 400 kilometers to the southwest (in 2007). Based on observations by Opportunity, this unit is interpreted to be a thin aeolian (wind-blown) mantle of basaltic sand and hematite concretions sitting on the etched terrain.
The etched terrain in this image is split into two units. The darker unit at the top of the image is filling in an approximately 120 kilometer NW-SE trending valley, while the brighter etched terrain in the middle of the image is stratigraphically and topographically higher than the lower etched terrain in the valley. This upper etched terrain is a plateau-forming unit with a geomorphic pattern that ranges from relatively flat plains to dissected plateaus and mesas. The lower etched terrain is flat with low albedo, and covered in dunes.
It is in these etched terrains that CRISM, and previously OMEGA, have detected hydrated sulfates, which makes a sedimentary origin seems most likely for these layered deposits of etched terrain found in Meridiani.
Written by: Jennifer Griffes (3 March 2010)
More info and image formats at http://hirise.lpl.arizona.edu/PSP_002324_1815
Image: NASA/JPL/University of Arizona