If this rock is as old as I think it is.....
...then present topography has nothing to do with the topography of deposition. In that cool stereo photo you posted, you can see that there are layers above the white layer. Now if that white layer is our white layer, then alot happened after the white layer was deposited. Including later erosion of the whole area.
The basin in which the Opportunity formation was deposited is long gone. The plain probably is there because this stuff is so soft, and so horizontal. Soft rocks tend tho form shelves, not cliffs, in arid weathering environments. Evidence of this ancient basin remains in the rocks there, but modern topography is not really a guide for where this basin was.