Martian crumbcake

I just can't get over the bedrock close-ups from the Opportunity rover. (Go here for the big version.) To me the rock still looks more like volcanic ashfall than waterlaid mudstone, but it could go either way. If the Moon is made of green cheese, then Mars is a pastry planet covered in windblown cinnamon. If you have an idea what this stone is, join the Forum and tell us.