Curious layered formations in recent Mars rover images

I tried to post this to sci.space.science on Sunday, where I thought it
would be most on-topic, but it never appeared, so I'm trying here
today:

Yesterday (Saturday) I was browsing the images at:
http://marsrovers.jpl.nasa.gov/gallery/images.html
where I found some really interesting images. Specifically:
Opportunity, raw images, Sol 278, navigation camera:
Thin layer of hard material (rock?), overlaying softer material.
Parallel grooves across top of unbroken hard layer long ago.
Later the hard material cracked into pieces, and softer material under
it is eroded away from under all the cracks, starting to undercut the
pieces of hard material.
In some images the pieces of hard rock have rotated from original
orientation, while other images they're still all parallel-grooved.
Does anybody have information about the kind of materials involved in
these images, the cause of the parallel grooves, and the process that
is involved in cracking and erosion through cracks?