This Petrified Wood is 225 million years old, from the Triassic Period. The trees fell and were covered with mud which kept oxygen from decaying them. Then silica in the ground water seeped into the wood, encasing it. The silica turned into quartz and then was exposed by erosion.

The Chinle Formation makes the stripes in the hills (Tepees). The reddish color near the bottom is caused by iron oxide and the darker red near the top is iron-stained siltstone. The white layer is gypsum. The varying shades of gray are decayed animals and plants. The hills were created by erosion.