Dreams of Summer: This is Where and When...Wait, They Made a National Park Out of This?

Why, yes. Yes they did. About two posts ago, I pointed out that there are some places where the geology is kind of...monotonous. One of those places is the vast sage plain east of Grand Canyon and Flagstaff, around the towns of Holbrook and Winslow (yes, that Winslow). The land is flat, windy and barren, hardly looking like a place of geological inspiration. And yet it is.

In the last post, we were stuck in a wayward time machine, emerging in a nightmarish swamp and river floodplain populated by strange and frightening reptiles and other creatures. I asked for help in explaining where and when we were, and the readers responded quickly and accurately (strangely enough, a bunch of readers share the same name: Anonymous!). We had landed in the Triassic Period, about 225 million years ago, in Petrified Forest National Park, Arizona.

Barren plains may be uninteresting to look at and explore, but they make for great transportation corridors, and the land that eventually would become Petrified Forest National Park was the site of a road in the 1860s, a railroad in the 1880s, and a national highway (the legendary Route 66) in the 1920s. The petrified wood was discovered by surveyor Amiel Whipple and geologist Jules Marcou, during explorations in the early 1850s (discovered by Europeans, anyway; there are ruins in the park made hundreds of years ago out of petrified wood!).

Once the railroad was constructed, travelers began to visit the area, and many removed huge amounts of petrified wood. There was even a business at one time that was prepared to ground up the "wood" for use as an abrasive. Local people began to fear that the precious mineral would disappear, and they began efforts to establish a national park to protect the resource. The first effort failed in 1895, but the passage of the National Antiquities Act in 1906 led to the proclamation of Petrified Forest National Monument the same year (Congress can establish parks, but the President can proclaim national monuments). It wasn't until 1962 that Congress established the area as a national park. In 2004, the size of the park was more than doubled, to 341 square miles (884 square kilometers). Despite the legal protection, it is estimated that tons of petrified wood are stolen every year. This, despite hefty fines and the easy availability of petrified wood outside the park boundaries.

I called the park plain and barren, but that's not at all true. The rocks here have been gently lifted and erosion has exposed the underlying shale, siltstone and mudstone layers into rapidly eroding badlands. Metals in the rock have oxidized into a veritable rainbow of colors. The northern regions of the park are known as the Painted Desert. The rocks are almost exclusively part of a single formation, the Chinle, representing a complex of river channel and floodplain deposits dating from the Triassic. Variations in the rocks have led researchers to organize the rocks in the park into four members: Mesa Redondo, Blue Mesa, Sonsela, Petrified Forest, and Owl Rock. The Chinle (or equivalent rock) is found throughout the southwest. There are something like 50 members identified, and the precise definition of the unit is controversial. To get a sense of the conflict, check out this overview: http://chinleana.fieldofscience.com/2008/11/chinle-confusion.html 

Petrified wood is one of the prettiest of Earth's minerals and rocks. The wood was originally buried in mud and volcanic ash. As the wood slowly decayed away, it was replaced by silica (the mineral quartz, SiO2), and stained by oxides of iron, manganese, copper, and other metals. The trees, some of which reached heights of 200 feet, are classed into at least three species, and as many as nine. Araucarioxylon arizonicum is the most common, and as such has been designated as Arizona's state fossil.

It turns out that Petrified Forest National Park preserves far more than fossilized wood. The extensive exposures of the Chinle have provided not dozens, but hundreds of fossil species, and not just plants. The Triassic Period was a pivotal time in the history of the Earth. It was established on the basis of a mass extinction that wiped out some 90% of the species living on the planet. The extinction decimated the dominant reptiles at the time, the mammal-like reptiles, allowing a group of diminutive archosaurs to gain ascendance in the ecosystem. The archosaurs include today's crocodiles and birds, and some relatively famous animals called dinosaurs. The late Triassic deposits of the Chinle contain some of the earliest dinosaurs known, including Coelophysis and Chindesaurus bryansmalli, a small carnivorous dinosaur found for the first time at Petrified Forest.

The rich fossil record and incredible exposures of the rocks of Chinle provide an important look at a time when the planet was getting back into some kind of ecological balance after the greatest catastrophe of all time. Petrified wood was the original attraction, but the park preserves a significant part of Earth's history.

The protection of the park has revealed another important resource. The explorations of the 1850s were hardly the first incursion of humans into the region. The presence of a secure source of water (the Puerco River) and a prairie environment populated by numerous grazing animals meant that people have lived in the region since the end of the last ice age 13,000 years ago. The park preserves hundreds of archaeological sites, including the ruins of entire villages.

The 2004 additions to the park didn't add all that much in the way of wood resources, but it did include a significantly large area of badlands wilderness in the Painted Desert. It used to be that no camping was allowed in the park, but there is a trackless region in the northwest part of the park now where one can got lost in the past.

So yes, they made a park out of this. For so many good reasons.

Time Beyond Imagining - A Brief History of the Colorado Plateau: Ash and Wood

Continuing on our journey through time on the Colorado Plateau, we explore the Chinle Formation, one of the most diverse and distinctive layers in the region. It forms most of the scenery at Petrified Forest National Park, and is ubiquitous in such widespread locales as Capitol Reef and Canyonlands National Parks, Grand Staircase-Escalante National Monument, Zion National Park, and Glen Canyon National Recreation Area. The formation was laid down during the late Triassic, a time of huge changes in the world, especially in the biosphere. It was the age of the earliest dinosaurs, but the world was populated by a host of unusual reptiles and amphibians. The Chinle was an ideal environment for preserving fossils: the shales, siltstones, sandstones and conglomerates were laid down in river floodplains and channels. The headwaters of the rivers were mountains far to the southeast, but a chain of volcanoes to the southwest provided vast amounts of ash to the landscape.

When the Chinle is sandwiched between cliff-forming layers, it forms distinctive lavender and red slopes, but is often covered by debris from the cliffs above. In parts of the Navajo Reservation and the region around Holbrook (known popularly as the Painted Desert), the Chinle forms spectacular badlands topography. The bright colors result from the oxidation of various metal ions in the clay layers, especially iron.

Map courtesy of Ron Blakey of Northern Arizona University

The trees of the Petrified Forest are the most famous fossils of the Chinle. Most of the trees accumulated as logjams in the large rivers that flowed northwest through the region. Some of the logs are as much as 100 feet long. Once buried in mud and ash, silica dissolved from the surrounding rock filled cell spaces or replaced the wood material entirely. Once again, metal oxides provide the striking colors, primarily reds and yellows.

It is pretty amazing that any petrified wood remains in the park. In the early part of the century, trains would stop nearby so people could carry off samples. At least one mining venture sought to grind the wood up into abrasives. Every year park visitors carry off more, but I notice that the National Park Service doesn't discourage the rumor that bad luck befalls those who steal wood. Ironically, people pay huge fines for stealing something that can be picked up free and legally outside the park. Some of the local rock shops even give away free samples.

The Chinle Formation also contains a rich vertebrate fossil record. Phytosaurs and metoposaurs are commonly found, as well as Coelophysis, one of the earliest carnivorous dinosaurs. Hundreds of their remains have been discovered at Ghost Ranch in New Mexico. Plateosaurus and Chindesaurus are two relatively small ancestors of the giant sauropods that dominated the later Mesozoic. A rich assemblage of other fauna and flora have been discovered. A nice review of the paleontology of the Chinle can be found here.

The Chinle has another heritage, a far more insidious one...our next post will explore this in more detail.

Mid-week Mystery Sample: What is it? Heck if I actually know!

Well, that just beats all. We community college instructors often revel in the fact that we know a little bit about a lot of things instead of a lot about some specific things, but sometimes it helps to actually know a lot. Back when we found the mystery sample of the Chinle Formation, we wondered if it was a coral, and how a coral could have gotten into the Chinle, which is a terrestrial formation.

As I described in the previous post, I thought I had discovered the answer to the mystery when I saw a metoposaurus in the visitor center at Petrified Forest National Park (shown in the photo above), given the unique pits in the skull plates. The metoposaurs were large carniverous amphibians. I couldn't exactly make my sample line up with any particular part of the skull, but it was an adequate answer for me, the non-expert. In the best tradition of bad science, I didn't continue to explore other hypotheses once I had an answer that satisfied me. That's the kind of attitude that drives me nuts when my students are trying to identify rock and mineral specimens ("it's clear so it must be quartz, even if it cleaves").

Of course, running a blog demonstrates your ignorance to anyone who wants to pay attention, so I say thank you to an actual paleontologist, Jerry Harris, who suggested my sample could also be a phytosaur scute. So I am properly chastised for not covering my bases (y'all notice I could have said I knew it was a phytosaur all along....). I am at the same time happy to find that I might have a piece of phytosaur. They were one of the fierce predators of the Triassic: somewhat crocodile shaped, up to 30 feet long, and overall pretty cool.

Picture from Wikipedia

(This post has been revised)