Pack up your stuff kids! We're going to a NATIONAL PARK!
Cool! Does it have big mountains?
No.
Oh, does it have deep canyons?
No.
Can we go swimming in a lake or river?
No.
Oh, will rangers give an interesting talk around a campfire?
No, there's no campgrounds.
Does it have trees at least?
Well, yeah, but they're all dead.
What kind of a national park is this???
It is of course the national park dedicated to a bunch of long dead trees, Petrified Forest in Arizona. It was the third day of our recent exploration of the Back of Beyond, the vast geological wonderland of the Colorado Plateau. We had traveled east of Flagstaff to explore the prairies and badlands that make up much of that strange landscape.
To most folks it's all about the petrified wood. Some might even think it's a one-note kind of place, of some interest maybe, but not worthy of a full exploration. They couldn't be more wrong! The park is a showcase of geology, archaeology, and paleontology. And it has some unique and dramatic scenery to boot.
The wood was discovered first by indigenous people thousands of years ago (evidence indicates a human presence in the region for at least 12,000 years). They made use of the petrified wood as projectile points, knives, scrapers, and even building materials (there is a reconstructed pueblo in the park made almost entirely of the agatized wood).
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| Petroglyph at Newspaper Rock in Petrified Forest National Park |
The European colonists who always get credited for their "discoveries" came along in the 1850s. The military investigated the region first, followed by ranchers and homesteaders. The area became better known as railroads were built through the region, taking advantage of the relatively flat terrain and availability of water along the Puerco River. A hotel stop at Adamana allowed passengers to tour what was then known as Crystal Forest, and walk off with tons of petrified wood samples. Later on a stamp mill was constructed to grind up the wood to be sold as an abrasive. It concerned some people.
Often the creation of a national monument, which can be established by presidential proclamation, is a controversial affair. Such actions were taken by presidents to protect endangered resources and archaeological sites that locals were often profiting from. It's only as tourism follows that proclamation that local folks begin to accept the presence of a monument in their midst. The story seems to be somewhat the opposite in this case, as the Arizona legislature unsuccessfully proposed a Petrified Forest National Park as early as 1895 in response to local concern about the disappearance of vast amounts of petrified wood. The passage of the Antiquities Act in 1906 gave Theodore Roosevelt the power to designate Petrified Forest as the nation's second national monument that same year. Congress waited until 1962 before "upgrading" the monument to national park status (monuments have the same level of protection that national parks do, although funding sometimes lags behind). Subsequent actions in 2004 more than doubled the size of the park to 341 square miles (884 sq km).
Despite the protective actions, it's estimated that 12 tons of petrified wood disappear from the park every year (the thieves need to read up on the curse of stolen wood). It's ironic that so much is stolen, since it can be purchased legally just outside the park.
Being petrified is not the normal fate of most trees. Most of them decay and are destroyed, but in just the right circumstances petrifaction can happen. The tree has to be isolated from the atmosphere and destructive microorganisms by being buried quickly in fine mud and clay. There needs to be a source of silica (silicon dioxide), and the movement of groundwater. At Petrified Forest, the trees were ripped from distant forests by muddy flashfloods and volcanic mudflows and transported many miles, coming to rest in river floodplains and swamps. Layers of volcanic ash provide the silica and the swamps and rivers provide the groundwater. Over time the silica fills in the cells of the wood, preserving it. Oxides of iron and other metals provide the bright coloration.
But that's not what I'm really here to talk about!
The real value of Petrified Forest National Park lies in the story contained in its multicolored strata, the Chinle formation. It records a tumultuous period in Earth's history, the middle to late Triassic Period from about 225 to 208 million years ago.
When the Chinle formation is exposed to the elements, the mud and clay is easily and quickly eroded, preventing the development of soil and inhibiting plant growth. The resulting barren desert badlands hardly look like the remains of a tropical environment of large rivers and lakes, but that is what it is. And those barren sediments hide a wealth of paleontological remains (that's short for "fossils").
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| Exposures of the Chinle formation in the northern portion of the park |
The Mesozoic Era (252-66 million years) is often called the Age of the Dinosaurs, and of its three periods, the Jurassic is best-known (perhaps because of a certain film series), and the Cretaceous only slightly less-known, since no one ever made a movie called "Cretaceous Park". It did however gain notoriety as the ending curtain-call on the reign of the diverse and occasionally gigantic reptiles, due to a massive asteroid strike in what is today the Gulf of Mexico.
But the Triassic Period gets less attention. There are few if any dinosaurs from the Triassic in children's plastic toy collections, and yet the Triassic is the period in which the dinosaurs appeared on the planet for the first time. Few of them were large, and they didn't dominate their habitat the way their progeny in the Jurassic and Cretaceous did. There were other creatures inhabiting their world, all of whom were survivors of the greatest extinction event ever to strike our planet. And from within their ranks the first mammals emerged.
The Permian-Triassic mass extinction event was the worst devastation ever visited upon life on planet Earth, wiping out as much as 80-90% of existing species. The cause is not precisely known, but a vast outpouring of lava and ash in Siberia is thought to be a main contributor to a sudden change in the climate that caused a spike in temperatures and acidity in the oceans (carbon dioxide rose from 400 to 2,500 ppm, compared to our current historical rise from 280 to 420 ppm). A total break-down in ecosystems followed, both on land and at sea. It took millions of years for the survivors to rebuild any kind of stability (only to be disrupted again at the end of the Triassic).
And this is why Petrified Forest is so important. It gives us a picture of conditions in the middle and late part of the Triassic when some semblance of stability had returned to the world. The Chinle formation, which is exposed over most of the park, has yielded up to 200 species of plants and around 60 species of animals. Nearly a hundred of them were first discovered at Petrified Forest National Park.
The Triassic world looked little like the subsequent Mesozoic era. The Jurassic and Cretaceous periods saw the dominance of terrestrial habitats by hundreds of species of dinosaurs big and small, plant-eaters and meat-eaters alike. Mammals and smaller reptiles scurried in the underbrush while small amphibians continued to thrive in rivers and streams.
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| Diorama of animals and plants of the Chinle formation, Petrified Forest National Park |
The Triassic was characterized by several animal groups that competed with each other in the aftermath of the great Permian extinction. There were still large predatory amphibians up to 10 feet long, including Metoposaurus. Many years ago, one of my students found an odd specimen in the Chinle (outside the park, for the record). It looked so odd that my guesses ran the gamut from coral to alien, but it wasn't until I saw this exhibit at the park museum that I thought of Metoposaur (see a picture here and here). To be fair, some paleontologists thought of phytosaur scutes as another possibility (see below).
There were some "not-quite-reptiles" called therapsids that were once called "mammal-like reptiles" but they actually represent a distinct group of organisms that shared a common ancestry with the reptiles. They were destined to become the ancestors to the first mammals later in the Triassic. In Permian time, the Therapsids and their related groups dominated terrestrial environments with hundreds of species, but they were decimated during the extinction. A few survived into the Triassic, including the Placerias hesternus, seen below. It was a plant-eater which at up to 11 feet and a ton in weight, was one of the larger creatures of the time.
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| Placerias hesternus, a dicynodont therapsid, a large plant-eater |
The pterodactyls made their first appearance (and no, they aren't dinosaurs either; they had a common ancestor with them). There were in fact some dinosaurs too, but they were mostly small, and most certainly didn't dominate their ecosystem. Some found in the park, like Coelophysis and Chindesaurus, were predators but being 3 to 7 feet long they were hardly the gigantic terrifying creatures of the Jurassic and Cretaceous periods.
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| Artist: Dr. Jeff Martz, NPS |
The creatures that brought terror to the ecosystem were the Archosaurs, the reptilian line that led to the crocodiles, birds and dinosaurs. The sediments of Petrified Forest National Park have yielded up specimens of Postosuchus, which at upwards of 23 feet in length, was gigantic by Triassic standards. It was a formidable predator.
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| Artist: Dr. Jeff Martz/NPS |
Another member of the archosaurs found in the park are the Phytosaurs. They strongly resemble the crocodiles, but are not that closely related. They are instead a marvelous example of convergent evolution, by which different lineages evolve to similar shapes (think sharks and dolphins).
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| Artist: Dr. Jeff Martz, NPS |
Petrified Forest is far more than a collection of petrified wood pieces lying around. It's a very different world and a fascinating place to visit and explore. It's one of the best places in the world to understand the time that gave rise to the mammals, and as such, to us.
