Exit 311 south from I-40 takes you through the Painted Desert and into Petrified Forest National Park, ending at US Highway 180. Then turn right and you'll get back onto I-40 at Holbrook to continue your journey. I've included photos of the Painted Desert at the end of this post.
The petrified logs look like someone cut them with a saw because petrified wood is mostly silica, also called quartz. After petrification but while the logs were still encased in matrix rock, the logs cracked under stress. As the logs eroded out from gravity and ice wedging, the cracks widened and segments separated. Silica naturally breaks on a clean angle.
The park has what is considered one of the largest concentrations of petrified wood in the world, with other large collections found in North Dakota, Argentina, and Egypt. Radioisotopic dated zircons tell us that the trees in the Black Forest were deposited about 211 million years ago and those in the Blue, Jasper, Crystal, and Rainbow Forests were deposited around 218 million years ago.
This was a large river system with galleries of trees along the waterways. As the trees died naturally over many years, some floated downstream to form log jams. The various “forests” in the park are those log jams: Crystal Forest, Jasper Forest, etc. The original national monument was created where the highest concentration was located in the large expanse of petrified wood deposit.
The logs are all lying down and thus were not preserved in a standing position. Most of them were transported for some distance before being buried. There are rooted stumps that can be found in the backcountry of the park.
These forests were of coniferous trees, tree ferns, and some gingkoes, and about a dozen types of petrified wood have been formally described. There are probably more species that have yet to be identified. Petrified Forest National Park protects less than 20% of the petrified wood in northeastern Arizona, with petrified wood also found on state land, Bureau of Land Management land, Navajo Nation land, and privately owned property. The wood being sold in regional gift shops comes from the private property, which can be collected by the owner, or by people with permission from the owner, and sold. Poaching from the national park is a problem.
The rings we see in some of the petrified wood do not represent annual growth rings. Since these were all sub-tropical to tropical trees, they probably grew year-round. There would need to be a growing season versus a non-growing season to create annual growth rings.
Two trees in the Long Logs area of the Rainbow Forest measure 137 and 141 feet long. This indicates that some of the trees may have approached at least 200 feet tall when alive.
The various colors represent the trace minerals in the quartz. Iron and manganese account for much of the coloration, carbon also can add black, and—rarely, there is chromium that provides a true green.
It is used as a semi-precious gemstone in jewelry and as an ornamental stone in book ends, clocks, furniture, etc. Petrified wood is valuable scientifically as fossils from around the world. Much of the petrified wood of the park preserves the original cellular structure of the wood allowing paleontologists to identify the wood type and to create climatic reconstructions for the Triassic Period.
Settlers built structures using the petrified wood as seen below...
Visitors can see much from the road and overlooks, but even more can be experienced by leaving the road and hiking the short maintained trails like the Blue Mesa Trail in the photo below. Or you can even venture into the backcountry wilderness area for several days.
The Red Basin Clam Beds have recently been opened for visitation along a hiking trail...
A number of species of wildflowers thrive in this environment from March to October. The plants of the park are pictured and explained here. The wildlife found in the park is pictured here.
The Painted Desert of Arizona
Abutting (and administered as part of) Petrified Forest National Park is the Painted Desert, basically a broad region of rocky badlands much like you see in South Dakota's Badlands National Park. It stretches over 93,500 acres and features deep lavenders, rich grays, reds, oranges and even pink rocks, a painting with chemicals by Mother Nature over the eons, using as Her tools earthquakes, floods, volcanic eruptions, and lots of sunlight.
Deposits of clay and sandstone, stacked in elegant layers, play with the setting Arizona sun in an altering display of colorful radiance. Both the Navajo and Hopi people have lived in the region for hundreds of years, but it was Spanish who gave this landscape the name we know by calling it "El Desert Pentado."
Some of this terrain is protected within the Petrified Forest wilderness area and some of the land is within the Navajo Nation.
Th entire Painted Desert terrain features extend nearly 120 miles in length and 60 miles in width, a total of approximately 7500 square miles!
Some have called this terrain a "multi-colored birthday cake."