The riverbank opposite the side I live on. The river was running high the last few days, as we have had lots of unseasonable rain. The white specks in the sandstone are clam shells. I find them in the chunks of sandstone broken off each year by erosion.
One year I had a complete clam, both halves of the shell, looking like I had just dug it up. I carefully brushed away the sandstone and set it on the porch. Later that same day I walked by and it had crumbled away to a pile of white powder.
Clearly not petrified, but it has been a very long time since these clams were living creatures.