Today's Mars Photo is of Tisdale-1 as taken by the Mars Exploration Rover Opportunity on Sol 2694. The right side of Tisdale-1 seems to be very smooth, though broken up by the veins that scar the entire rock. This implies a high level of heat (likely from impact by the meteorite that created Endeavour Crater) may have fused different rocks together, thus making this a composite rock like the other Tisdales, which is likely why it is in that category in the first place. The veins on the surface throughout imply some form of erosion that took place after the formation of the rock, but by what we can only speculate.
Click the image to see the enlarged version from NASA.