Actually, Tom, I put a different interpretation on it. The heat shield impact scar is a very low-relief feature. Sand and spherules have been scattered from it, and pieces of pale sulphate rock blasted out. The loose cover seems to be about 6-12" deep, max. Below that is iron-hard rock upon which the heatshield destructed.

All across the plains we see traces of cracks (like the fissure called 'Anatolia' - not the region on earth where pedestal rocks occur!). We see "ghosts" of buried slabs from time to time.

IMHO, the whole area is a rock pavement, thinly covered by a gravel of spherules. If any of the soft sulophate rocks become exposed, they are very vulnerable to erosion and are removed fairly quickly. It is only hard "erratics" (to borrow and abuse a glacial term) like 'Bounce' and 'Heat Shield Rock' that can resist the erosion long enough to be observed today.

Look at degraded craters like 'Fram' - which had scattered debris that was weathering fast, and around Endurance and Eagle craters, loose debris had been removed and all that remained was a paving-slab texture with everything stripped-off at a smoothed average ground level.

Erosion here is fast until you get left with nothing but spherules, and then it's damn slow. (~100m in 1+ billion years = 1mm in 10,000 years)