good question, actually
My guess is that the upper rock unit (the Opportunity Formation, whatever the outcrop is) is regionally horizontal and covered by an unknown thickness of the weird spheroid rich soil. The only place we'll see it is in trenches or craters. I'm looking forward to seeing what is deeper in the stratigraphy.
Spirit, however, is traversing a big, undulating and broken-up series of mafic lava flows.
Just my quick thoughts....