The Pancam images of "Flat Rock" which caused the RAT to stall look a lot less crumbly than those of the prior white outcrops. Perhaps it is actually a harder rock:

Compare:

Panoramic Camera :: Sol 035 (73 images)
http://marsrovers.nasa.gov/gallery/all/opportunity_p035.html

to:

Panoramic Camera :: Sol 045 (89 images)
http://marsrovers.nasa.gov/gallery/all/opportunity_p045.html

Perhaps this can be tested by seeing how deeply the rover wheels can grind into each of these rocks.
Someone also suggested here that there is a statification of different types of evaporites on Earth, with carbonates lying below sulfates. If a trench can be ground into these rocks maybe this stratification can be observed.

Bob Clark