We were out hiking one day last spring when I decided to rest on a large boulder while the boys played in a nearby stream.  As I got ready to sit down, I noticed some formations in the rock, pictured below.  I was certain that I had come across some fossils, but in fact they were Liesegang bands, which form when iron in the water diffuses through a porous rocks, like the sandstone boulder I was about to sit on.  In this instance, the ferrous oxide has been deposited along the fractures in the stone to give it a sort of honeycomb appearance.  The last picture was set as my desktop image for a few months.