Really old logs!

In Petrified Forest NP you see old logs lying all over the place. From a distance, they look like ordinary wood, but in reality they're rocks that are over 200 million years old.

The bottom line is that this was a lush forest back in the Triassic era, when the earliest dinosaurs hung out. The trees fell in wet areas where they were covered with sand and mud with a silica content, and the silica gradually replaced the wood in the petrification process. I don't remember exactly how it works, but of course Wikipedia can supply the details. Or better yet, visit the park!

In some cases such as the photo of the individual tree, you can still see the rings quite clearly.