Shrunken heads at Endurance Crater (MER)

Have a look at the structures in this image taken Thursday by Opportunity.
They look like what is left over when you pull the forms off a cement slab.
Although incredibly fragile looking, they stand upright and are lined up
almost perfectly linearly. Reminiscent of the Easter Island sculptures in
miniature. Any idea what might have caused them. Perhaps a fracture from
the impact that formed the crater? Erosion by water or maybe sediment
caried by water and slapped up against a flat rock? It sees like they were
up against something of different composition with a straight edge that is
now gone.

http://marsrovers.jpl.nasa.gov/gallery/all/1/p/162/
1P142569631EFF3221P2387L7M1-BR.JPG