bedrock hills?

As Spirit gets closer to the hills, the Pancam resolution is delivering more and more detail. The latest images suggest to me that there are some fairly prominent bedrock outcroppings straight ahead.

Over at Meridiani, the Opportunity rover's got me a bit confused. How did this happen? Could it be that the rover's wheel lifted up a chunk of clotted or crusty surface dirt, rolled under it, and then deposited it right back in place? How else to explain the abrupt disappearance of the rover track? Was this a trenching maneuver?

Just for fun, I put together a quick panorama of some Opportunity Pancam pictures from sol 110. Click the image below for a higher resolution, uncropped version (still only 50% the resolution of the originals which can be found at the MER mission pages.)

My bandwidth is limited, so if you're not terribly interested in Mars (or my photoshop efforts) then save some of my quota for those who are and don't click through to the larger image. Thanks. If you're interested in the full resolution image, let me know.

update: So it's a rock. That makes sense. Thanks to Dan and Ben, I think I see what's going on here a bit better. Here's a section (slightly lightened) of the image Ben mentioned:

It pretty clearly shows that the wheel did cut across the corner of the dirt-covered slabby, flat rock. I wonder if it did lift the rock significantly or if it just disturbed it enough to define its edges. I still think it's an odd picture.