The amount of time that a geologic deposit is exposed at the surface can be measured by counting the number of impact craters that it contains. The longer a deposit is exposed at the surface the more impact events that it endures.
In this image, there are at least two distinct geologic units, a lighttoned bedrock and a surface veneer of darktoned material that contains sand dunes. The lighttoned bedrock must be older that the darktoned veneer of sand; the bedrock must have been present first in order to be covered by the sand. The darktoned sand however, contains many more impact craters than the lighttoned bedrock. This suggests that the surface of the bedrock is younger than the veneer of sand.
This can be explained by the bedrock being more easily eroded by the wind than the veneer of sand. The surface of the bedrock is rapidly refreshed (craters smoothed away), while the sand veneer retains impact craters for a longer period of time. This indicates that the bedrock is very friable (weak and easily eroded, in this case by the wind) and the sand veneer is less friable.
Written by: Chris Okubo (20 October 2010)
This is a stereo pair with PSP_002550_1725 .
More info and image formats at http://hirise.lpl.arizona.edu/PSP_003618_1725
Image: NASA/JPL/University of Arizona