Phil,
Check out the color version of the Mars panorama.
The skid marks from the air bags go through a slight sandy depression in the ground.
See
http://marsrovers.jpl.nasa.gov/gallery/press/spirit/20040112a/mspan_2X_final-A10R1.jpg (WARNING if you're on low bandwidth you'll be stuck a long time...it's a huge color file).
Look in the sandy hollow for the bouncing pattern of skid marks. Then, look towards the "back and right" of the sandy depression. There appear to be several dozen rocks of different albedo and origin. What's your speculation of those rock types?
