Anyway, whatever it is, it looks like the crater that Opportunity is in was excavated from sedimentry rock.
In this image, at the far right end there’s a block of the rock on show which looks like it has cross bedding. Such things don’t happen in igneous rocks, only sedimentry ones. Now, this doesn’t mean that there is water on Mars. Cross bedding can be created in sediments which are sub-aerial, sand dunes, but they can also be generated in sub-aqueous environments, namely flowing water, sediment ripples.
From the size of the bedding planes in this bedding it looks more like sub-aerial sand dune type deposits such as can be seen in the cliffs around Dawlish Warren.