How does wood turn to stone? Around 225 million years ago, in the late Triassic, receding river-valley floodwaters left thousands of what then were tropical-forest logs in deep mud, where they were buried, and in turn, protected from rot. Meanwhile, they slowly infused with silica minerals from layers of volcanic ash that fell on top of the surrounding sediment. Major erosion of the log-bearing Chinle formation began about 60 million years ago, when this area of the continent rose several thousand feet. Soft Chinle rocks wore away from the hard, water-resistant silica that took the place of wood in the logs. Now we have nearly perfect casts of logs in colorful mineral forms to admire and photograph, in a high desert where very few live trees can be found.
