The Petrified Forest holds the largest and most beautifully preserved collection of petrified wood ever found. Here, the slow process of fossilization has turned large trees into solid stone. Some 225 million years ago, these trees were part of an ancient forest, which was home to giant fish-eating amphibians, large reptiles, and early dinosaurs. After falling, the trees were washed downstream onto a flood plain at this location in northeast Arizona, and subsequently buried by silt and volcanic ash. Many of these trees rotted away, but the ones that survived were transformed into the beautiful fossilized logs we see today. Dissolved silica from the volcanic ash slowly filled or replaced the cell walls, crystallizing the trees into mineral quartz.
The process was often so precise that it preserved every detail of the log surface and, occasionally, the internal cell structures. Iron-rich minerals combined with quartz during the petrification process, to give the trees a brilliant rainbow of colors. Today fossilized logs lie strewn across the clay hills and exposed in cliff faces. The petrified trees are hard and brittle and break easily when they are subjected to stress.
The park is a window to the past. Besides the trees, it has also preserved a wonderful collection of dinosaurs from the Triassic period, when the “Age of Dinosaurs” was just beginning. Visitors can see these fossils in the Rainbow Forest Museum, alongside the giant reptiles and giant amphibians that once called this place home. The Petrified Forest also contains many fine examples of rock art which early people carved on to the surfaces of boulders, canyon walls, and rock shelters. The range of images is staggering: human forms, feet and handprints, cougars, birds, lizards, snakes, bats, coyotes, bear paws, bird tracks, cloven hooves, and numerous geometric shapes. These petroglyphs may commemorate important events, mark clan boundaries, document natural events such as the summer solstice, and some may even be doodles.
The park also has a climate of extremes— half of the 10 inches (250 millimeters) of annual rain arrives via violent thunderstorms in July, August, and September.