Natural World

Fossils

Fossils are clues to the past, allowing researchers to reconstruct ancient animals and plants. Together with the surrounding rocks, fossils tell the stories of ancient ecosystems.

Petrified Forest has plant and animal fossils representing the Late Triassic, going back over 200 million years. The Triassic Period is sometimes referred to as the "Dawn of the Dinosaurs" because it is the time before the Jurassic Period when dinosaurs ruled the earth.

Fossils can be big - some petrified logs found in the park measure over 190 feet - or small - a single tooth less than an inch long may contain the definitive characteristic of a new species.

Every fossil found is like a piece in a jigsaw puzzle. Imagine putting together a jigsaw puzzle without knowing what the completed puzzle will look like. That would be hard, but what if you also didn't have all the pieces?

Context is very important with fossils. Where a fossil is found, what else it is found with, and how it was preserved are clues just as important as the fossil itself. Every fossil found in the park is important because of its context. The context tells its story.

When visiting the park, enjoy the petrified wood, but leave it where you find it. The park loses several tons of petrified wood each year through the acts of thoughtless visitors.

As part of the Petrified Forest National Park Learning Center, a monthly lecture series is held the first Wednesday of each month. Presentations will feature paleontology, geology, archeology, and other topics. Lectures are free and open to the public.

For more information on fossils, including educational resources for K-12 teachers, check out The Paleontology Portal website.

For more in-depth educational and scientific information about fossils visit the Society of Vertebrate Paleontology website, representing professionals, students, artists, preparators, and others interested in vertebrate paleontology.

Petrified Wood

Petrified wood found in the park and the surrounding region is made up of almost solid quartz. Each piece is like a giant crystal, often sparkling in the sunlight as if covered by glitter. The rainbow of colors is produced by impurities in the quartz, such as iron, carbon, and manganese.

Over 200 million years ago, the logs washed into an ancient river system and were buried quick enough and deep enough by massive amounts of sediment and debris also carried in the water, that oxygen was cut off and decay slowed to a process that would now take centuries.

Minerals, including silica dissolved from volcanic ash, absorbed into the porous wood over hundreds and thousands of years crystallized within the cellular structure, replacing the organic material as it broke down over time. Sometimes crushing or decay left cracks in the logs. Here large jewel-like crystals of clear quartz, purple amethyst, yellow citrine, and smoky quartz formed.

Most of the petrified trees have been given the name Araucarioxylon arizonicum. Woodworthia and Schilderia are two other species occuring in small quantities in the park. Though only seven species of tree have been identified through petrified wood, over 200 species of plants have currently been identified from other Triassic fossils, such as leaves, pollen, and spores.

Petrified trees today lie strewn across clay hills and within cliff faces; each log broken into large segments. The quartz within the petrified wood is hard and brittle, fracturing easily when subjected to stress. During the gradual uplifting of the Colorado Plateau, starting about 60 million years ago, the still buried petrified trees were under so much stress they broke like glass rods. The crystal nature of the quartz created clean fractures, evenly spaced along the tree trunk, giving the appearance today of logs cut with a chainsaw.

NATURE & SCIENCE

Petrified Forest was set aside as a national monument in 1906 to preserve and protect the petrified wood for its scientific value. It is recognized today for having so much more, including a broad representation of the Late Triassic paleo-ecosystem, significant human history, clear night skies, fragile grasslands ecosystem, and unspoiled scenic vistas.

Scientific studies are on-going at the park. Paleontologists find new fossils, including new species of plants and animals, each year. Biologists study living plants and animals, including vegetation surveys and reptile, amphibian, and mammal projects. Archeological site monitoring is on-going. Air quality, weather, and seismic monitoring stations constantly generate new data.

Plant Megafossils

Late Triassic plant megafossils found in Petrified Forest National Park
Compiled by Dr. Sid Ash, September 2003

* species based on fossils collected in the park
** species and genotype based on fossils collected in the park

Lycopods
Chinlea campii**
Chinlea sp.

Horsetails
Equisetites bradyi*
Equisetites spp.
Equicalastrobus chinleana**
Neocalamites virginiensis
Neocalamites sp.

Ferns and Fernlike Foliage
Cladophlebis daughertyi*
C. yazzia*
Cladophlebis spp.
Clathropteris walkeri*
Cynepteris lasiophora*
Itopsidema vancleaveii**
Phlebopteris smithii*
Sphenopteris arizonica*
Todites fragilis*
Wingatea plumosa**

Cycads
Aricycas paulae**
Charmorgia dijolli**
Lyssoxylon grigsbyi**

Cycadeoids
Zamites powellii

Ginkgoes
Baiera arizonica*

Cordaites
Dadoxylon chaneyi*

Conifers
Araucariorhiza joae**
Araucarioxylon arizonicum

Brachyphyllum hegewaldia*
Pagiophyllum simpsonii*
Podozamites arizonicus*
Samaropsis puerca*

Position Uncertain
Carpolithus chinleana*
Dinophyton spinosus**
Marcouia neuropteroides**
Pramelreuthia yazzi*
Schilderia adamanica**
Woodworthia arizonica**

Nonnative Species

The National Park Service places a high priority on encouraging, restoring, or maintaining plant and animal diversity in all of its sites across the country. One of the fronts in the battle to maintain healthy, balanced ecosystems is waged against invasive species.

Nonnative species (also referred to as invasives, exotics, introduced, or nonindigenous species) have been introduced intentionally or unintentionally into new ecosystems. An invasive species is a nonnative whose introduction causes or is likely to cause economic or environmental harm or harm to human health. Invasive species take over habitat, squeezing out the native flora and fauna. This reduces biodiversity, interferes with soil productivity, damages or replaces native populations, and changes land and water quality. Each year invasive plants cause billions of dollars in damage to public and private lands and the ecosystems upon which we all depend. In the National Park Service, 196 national park areas have serious problems posed by invasive plant species. The costs of managing weeds were estimated at $80 million from 1996-2000.

Of particular concern on the Colorado Plateau are plants which are invading rangelands and waterways. Control of infestations has been difficult and the ecological consequences have been serious. Invasive weeds which are of particular concern include camelthorn, Canadian thistle, Russian knapweed, cheatgrass, common purslane, and field bindweed. These plants grow where the earth has been disturbed and little competition for resources exists. They establish root systems and grow strong enough to expand aggressively. Some have seeds which can lie dormant for many years, even up to half a century. Others have extensive horizontal root systems which can spread rapidly over large areas. They may germinate in the fall and produce seeds in the early spring. The seedlings then have a considerable head start, stealing water from native seedlings which sprout later in the season.

Russian olives and tamarisk, or saltcedar, were introduced to the Southwest with the good intentions of providing ornamental plants and natural erosional controls. Unfortunately, these invasive species have taken over a lot of native habitat. They crowd out cottonwoods and willows at water sources, which in turn affects migrating and breeding bird species. The result has been a significant decrease in biodiversity and ecosystem health along much of the Colorado Plateau's waterways, including waterways within Petrified Forest National Park.

The consequences of introducing nonnative species are sometimes not recognized for generations, but rarely do the advantages outweigh the risks. Ongoing efforts are being made to take out invasive species throughout the parks. The National Park Service, in cooperation with a wide variety of local, state, and other federal agencies, is working to control invasive species through cooperative partnerships with communities and unifying management plans. Invasive species know no boundaries; the aliens are in everyone's neighborhood!

If you are a teacher or a student, you might be interested in an on-line program that will help you learn more about Invasive Plant Species. It's called Alien Invasions: Plants on the Move and is a K-12 curriculum-based program, developed by the Bureau of Land Management and scientists.

Kingdom Plantaea

Linnaean taxonomy of Late Triassic Plants of Petrified Forest National Park
Compiled by W.G. Parker and Sid Ash December 8, 2004

Kingdom Plantae
 Division Pteridophyta
Class Filicopsida
     Order Uncertain
       Family Guaireaceae
         Genus Itopsidema
           Species vancleaveii
       Family Cynepteridaceae
         Genus Cynepteris
           Species lasiophora
     Order Gleicheniales
       Family Gleicheniaceae (?)
         Genus Wingatea
           Species plumosa
     Order Matoniales
        Family Matoniaceae
      Genus Phlebopteris
            Species smithii
     Order Polypodiales
        Family Dipteridaceae
          Genus Clathropteris
            Species walkerii
     Order Osmundales
        Family Osmundaceae
          Genus Todites
            Species fragilis
     Order Hymenophyllales
         Family Hymenophyllacaea
           Genus Hopetedia
             Species n. sp.
   Class Uncertain
  Order Uncertain
     Family Uncertain
        Genus Cladophlebis 
          Species daughertyi
              Species yazzia
              Species sp.
           Genus Marcouia
              Species neuropteroides
Division Coniferophyta
Class Pinopsida
  Order Coniferales
     Family Uncertain
        Genus Araucarioxylon
              Species arizonicum
         Family Araucariaceae
            Genus Araucarites
               Species n. sp.
            Genus Araucariorhiza
               Species joae
         Family Uncertain
            Genus Brachyphyllum
               Species hegewaldia
            Genus Pagiophyllum
               Species simpsonii
            Genus Podozamites
               Species arizonica
               Species n. sp.
   Order Cordaitales
      Family Cordaitaceae
            Genus Samaropsis
               Species puerca
            Genus Dadoxylon
               Species chaneyi
   Class Uncertain
       Order Uncertain
          Family Uncertain
             Genus Schilderia
                Species adamanica
             Genus Woodworthia
                Species arizonica
             Genus Pelourdea
                Species sp.
             Genus Carpolithus
                Species chinleana
Division Pteridospermophyta
Class Uncertain
   Order Uncertain
      Family Uncertain
             Genus Sphenopteris
                 Species arizonicum
             Genus Dinophyton
                 Species spinosus
             Genus Pramelreuthia
                 Species yazzi
Division Sphenophyta
Class Equisetopsida
       Order Equisetales
          Family Equisetaceae
             Genus Equisetites
                 Species bradyi
                 Species sp.
             Genus Equicalastrobus
                 Species chinleana
             Genus Neocalamites
             Species virginiensis
                 Species sp.
 Division Lycophyta
Class Lycopsida
   Order Lycopodales
       Family Uncertain
           Genus Chinlea
              Species campii
               Species sp.
               Genus Lycopodites?
                   Species sp.
Division Cycadophyta 
Class Bennettitopsida
      Order Bennettitales
          Family Uncertain
              Genus Zamites
                   Species powellii
Class Cycadodsida
       Order Cycadales
           Family Uncertain
               Genus Aricycas
                   Species paulae
               Genus Cycadospadix
                   Species n. sp.
               Genus Lyssoxylon
                   Species grigsbyi
               Genus Charmorgia
               Species dijolli
 Division Ginkgophyta
Class Ginkgoopsida
   Order Ginkgoales
          Family Uncertain
               Genus Baiera
                   Species arizonica
               Genus Ginkgoites
                   Species n. sp.

Wildflowers

Wildflowers have adapted to the arid climate at Petrified Forest National Park in many different ways. Thick, waxy coverings on leaves and stems reduce evaporation. Small leaves reduce the effects of solar radiation and water loss. Deep taproots reach far into the soil to find water while shallow, widespread roots catch and absorb surface water quickly. Despite these adaptations, wildflowers avoid drought and heat by hiding in the soil as seeds or bulbs, sometimes for decades. Germination only occurs after significant seasonal rainfall.

Not all wildflowers bloom during the day. Some take advantage of cooler night temperatures to open their flowers. These evening-blooming plants include evening primrose, sacred datura, sand verbena, and yucca. The yucca and the yucca moth have a fascinating dependency on one another which includes trysts in the night. After mating, the female moth gathers pollen from a yucca flower and packs it into a ball. She then flies into the night until locating another yucca flower. Here she lays eggs in the base of the flower's pistil and packs pollen from her pollen ball down into the pistil, providing food for her young when they hatch. She visits several flowers during the night, each time distributing the pollen from flower to flower. Thus she fertilizes the yucca flowers while ensuring that her young will have nutrients to live on. Yucca plants and moths have a symbiotic relationship: yucca flowers are only pollinated by yucca moths and yucca moth larvae only feed on yucca pollen.

April and May, following winter snow and rain, are generally the best months to see wildflowers throughout the Southwest. Beginning in late August, if the summer monsoon season (July-August) has brought significant rainfall, different species bloom and color the landscape once again.

Grasses

Grasses are one of the most important plants within the grassland ecosystem found in the park. Large expanses of grasslands form where wind-blown sediment and erosion have created a layer of soil several feet thick. One of the most devastating causes of grassland destruction is grazing by cattle and horses. Because grazing is not allowed within the park, the area has returned to a more natural state and is one of the largest recovering grasslands in the Southwest.

Individual grasses sprout almost anywhere they can find soil, even in potholes filled with dirt. Most grasses fit into two basic groups, bunch grass and sod-forming grass. Bunch grass is classic arid adapted grass occurring in scattered clumps. Its spreading growth pattern reduces competition for limited soil nutrients and water. Examples of bunch grass include rice grass and needle-and-thread grass. The large rice grass seeds are rich in protein and were an important source of food for American Indians. Needle-and-thread grass has a sharp seed attached to a thin thread-like tail which develops in a spiral, wound fashion. As the thread unwinds, it drives the seed into the ground. Both of these grasses are perennial, becoming dormant during droughts. Rice grass plants have been known to live over 100 years, through wet and dry times.

Sod-forming grass is what most people have in their yards. Galleta and Blue Grama, sod-forming perennials native to Petrified Forest National Park, usually grow together. Cheat grass was accidentally brought to the United States in the 1800s and is now found throughout the park.

To visitors unaccustomed to the vast landscapes of the Southwest, Petrified Forest National Park can seem somewhat barren. On closer look, the land is teeming with interesting and clever life.

Plants

Not all plants at Petrified Forest National Park are fossils. Living plants are critical components within the grassland ecosystem found throughout the park. Plants capture particulate dust in the air, filter gaseous pollutants, convert carbon dioxide to oxygen, provide habitat for animals, and supply raw materials for humans.

Plants of arid climates have adaptations which enable them to survive the extremes of temperature and precipitation. These adaptations can be grouped in two basic categories, drought escapers and drought resistors. Drought escapers are plants which take advantage of favorable growing conditions when they exist, but go dormant when those conditions disappear. They are usually annuals, growing only when enough water is available. Seeds produced under good conditions can lie dormant for years if conditions are not favorable for germination. Most grasses and wildflowers are drought escapers.

Drought resistors are typically perennials. They have mechanisms for reducing the damage a drought can cause. For example, some will drop their leaves if water is unavailable. Many have small, hairy leaves which reduce exposure to air currents and solar radiation and thereby limit the amount of water lost to evaporation. Cacti, yuccas and mosses are examples of drought resistors. Yuccas have extensive taproots which can reach water beyond the ability of other plants. Mosses can tolerate complete dehydration. When rains return after extensive dry periods, mosses green up immediately.

Lichens

In many areas of Petrified Forest National Park, large collections of exposed rock surfaces are covered by lichens, especially the shaded north side. A lichen is actually a simple community of at least two mutually-dependent organisms: fungi and green algae. When both organisms are dependent on the other, they are said to be symbiotic. Green algae uses the photosynthesis process to produce food for the fungus, while the fungus protects the algae from the elements and extracts nutrients from soil and rock. The lichen structure is more elaborate and durable than either fungus or algae alone.

Lichens are well adapted to arid climates. They can continue food production at any temperature above freezing. Lichens can absorb more than their own weight of water and can absorb emphemeral water, such as dew, almost directly into their cells. The water does not need to go through roots and stems as it does in vascular plants.

Many other plants benefit from the presence of lichens. The green algae component of lichens can transform nitrogen in the air, which is unusable to most organisms, into a form which is essential for life. This is especially important in arid climates where lack of nitrogen is known to limit productivity.

Because lichens take everything they need from the air, they are dependent on good air quality. Scientists turn to lichens as indicators of air quality. It is truly amazing how nature, over time, can establish symbiotic relationships for the benefit of the whole ecosystem.

Birds

Like many national park areas, Petrified Forest National Park provides a unique place of protection and preservation. Here, birds can find food and shelter that may not be available in other regions on their journeys from habitat to habitat. Patches of healthy, undeveloped habitats are found in the modern fragmented landscape, connected by corridors such as Petrified Forest National Park.

Petrified Forest has a variety of habitats. Raptors, songbirds, and ground birds can be found in the grassland. Riparian corridors provide food and shelter for warblers, vireos, avocets, killdeer, and others. The exotic and native trees and shrubs around the Visitor Center and Rainbow Forest Museum provide home for western tanagers, hermit warblers, and house finches. The park also offers sightings of vagrant shore birds and rare Eastern birds not seen often in Arizona. Rarities, such as black-throated blue warbler, have been found by Maricopa Audubon Society members. September and early October seem to be the best time to visit the area to see these fascinating vagrants.

Like the canary in the mine, birds gauge the health and safety of our environment. By watching birds, noting species, and the migration of species, we can understand the changes in our environment.

Amphibians

Can you imagine living underground for nine months of the year and not eating, drinking, or defecating? Amphibians are an amazing group of animals do just that.

It is hard to imagine that in this dry region animals that require consistent moisture could thrive. Three hundred and fifty million years ago the first fish-like amphibian hauled itself out of the sea. Within the sedimentary rock of the park, giant amphibians such as metoposaurs have been discovered as fossils. By the time dinosaurs appeared, amphibians were flourishing. Today, they are still among the most successful groups of animals.

Why have they survived and adapted to such varied environments worldwide? Permeable skin! Amphibians do not drink; they absorb water through their skin. Spadefoot toads, residents of the park, absorb water from the soil in which they hibernate. Although permeable skin allows for water absorption, it provides little barrier to evaporation. This causes the animal's water balance to be in constant flux. Evaporative water loss also results in loss of body temperature. This is why you often see amphibians on warm pavement in the evening. It is not an easy life for amphibians in this dry grassland. Behavioral and physiological mechanisms that shape their daily life make it possible for them to survive.

Although amphibians have survived here for millions of years, today they are in trouble. Biologists around the world have noted dramatic declines in amphibian populations. No one knows what is causing these declines, but it is thought to be a sign of unfavorable environmental changes. Habitats such as wetlands are being destroyed, pesticides and metal poisons are contaminating the water, new predators are being introduced, the ozone layer is being depleted, and global climate changes are underway. In some cases, natural population fluctuations may explain the decline but scientists have ruled out natural causes as the only explanation for the overall problem. All around the world, declines are occurring in many species. What is clear is that human actions are the primary cause of these declines.

The following is a list of amphibians known to currently occur in the park. Further research will undoubtedly locate more species as different habitats in the park are more thoroughly studied.

Tiger salamander, Ambystoma tigrinum
Woodhouse's toad, Bufo woodhousii
Red-spotted toad, Bufo punctatus
Great Plains toad, Bufo cognatus
New Mexico spadefoot, Scaphiopus multiplicata
Plains spadefoot, Spea bombifrons
Couch’s spadefoot, Scaphiopus couchii

Animals

Animal life at Petrified Forest includes amphibians, birds, insects, spiders, mammals, and reptiles. Birds, lizards and rabbits are seen most frequently, though seasons and weather play a large role in determining what animals are active.

For many animals, activity occurs during a particular temperature range. "Crepuscular" animals are active at dawn and dusk, the coolest times of day. The half-darkness makes prey animals less visible, yet visibility is good enough to locate food.

Activity can change with the season, too. Snakes and lizards are "diurnal" (active during the day) in late spring and early fall, but they become "crepuscular" (active at dawn and dusk) during the heat of summer.

Many animals in the park are "nocturnal" (active at night). This is an adaptation not only to avoid high summer daytime temperatures, but also to avoid certain predators.

You are much more likely to see animal life in the park if you come as early as park hours allow and stay as late as allowed. These are also the times when the angle of the sun makes the views and colors of the Painted Desert most spectacular.

Whenever you are in a national park, do not approach, feed, or harass any wildlife. Help your parks reduce the impact of human visitors to the homeland of many wild species.

Mammals

Mammals are a diverse group of animals, ranging from the delicate white-footed mouse to the elegant mule deer. Mammals have fur or hair, produce milk for their offspring, and are warm-blooded.

In the often extreme climate of the plateau country, animals use such survival strategies as hiding in their burrows or migration as well as physiological adaptations like hollow hairs for insulation. Many animals in arid regions are nocturnal, using the cooler night to survive the heat of summer or the darkness of the late hours to escape notice of predators.

Early morning is the best time to view mammals while in the park. Do not approach, feed, or harass any wildlife in Petrified Forest or any other national park area.

The list below features only a few of the many species of mammals in the park.

Coyote, Canis latrans
Gray fox, Urocyon cinereoargenteus
Swift fox, Vulpes velox
Bobcat, Lynx rufus (Felis rufus)
Mule deer, Odocoileus hemionus
Pronghorn, Antilocapra americana
Ringtail, Bassariscus astutus
Raccoon, Procyon lotor
Badger, Taxidea taxus
Striped skunk, Mephitis mephitis
Western spotted skunk, Spilogale gracilis
Black-tailed jackrabbit, Lepus californicus
Desert cottontail, Sylvilagus audubonii
Desert shrew, Notiosorex crawfordi
Pallid bat, Antrozous pallidus
Townsend's big-eared bat, Corynorhinus townsendii (Plecotus townsendii)
California myotis, Myotis californicus
Fringed myotis, Myotis thysanodes
Yuma myotis, Myotis yumanensis
Western pipistrelle, Pipistrellus hesperus
Porcupine, Erethizon dorsatum
Gunnison's prairie dog, Cynomys gunnisoni
White-tailed antelope squirrel, Ammospermophilus leucurus
Spotted ground squirrel, Spermophilus spilosoma
Rock squirrel, Spermophilus variegatus
Botta's pocket gopher, Thomomys bottae
White-throated woodrat, Neotoma albigula
Bushy-tailed woodrat, Neotoma cinerea
Mexican woodrat, Neotoma mexicana
Stephens' woodrat, Neotoma stephensi
Ord’s kangaroo rat, Dipodomys ordii
Silky pocket mouse, Perognathus flavus
Northern grasshopper mouse, Onychomys leucogaster
Brush mouse, Peromyscus boylii
Canyon mouse, Peromyscus crinitis
White-footed mouse, Peromyscus leucopus
Deer mouse, Peromyscus maniculatus
Pinon mouse, Peromyscus truei
Western harvest mouse, Reithrodontomys megalotis
House mouse, Mus musculus

Natural Features & Ecosystems

Petrified Forest National Park contains the petrified remains of 225 million year old trees from the Late Triassic. Surrounding the petrified wood are millions of years of deposition, uplift, and erosion, creating the Chinle Formation. This rock formation creates the red hues of the Painted Desert and the blue tones of the Blue Mesa region.
 
Petrified Forest is situated near the southern edge of the Colorado Plateau with elevations ranging from 5300 feet to 6235 feet. It was the uplift of the Colorado Plateau, starting about 60 million years ago, and the erosion that followed and continues today, which carved the present landscape.

The colorful mudstones and clays of the Painted Desert badlands are composed of bentonite, a product of altered volcanic ash. The clay minerals in the bentonite can absorb water to as much as seven times their dry volume. The expansion and contraction properties of the bentonite cause rapid erosion by preventing much vegetation from growing on the slopes of the hills.

Other prominent features created by erosion are mesas and buttes. Both have flat tops of more erosion-resistant sandstone over softer clays. Mesas are quite broad but not very tall, while buttes are taller and more narrow. In this picture sandstone caps the top of an eroding mesa. The sandstone is more erosion resistant than the claystone underneath. Eventually the harder rock will erode away, leaving the softer claystone underneath exposed to the elements. This will then become another rolling bentonite hill within the badland landscape.

Geologic Formations

Erosion has sculpted and shaped intriguing landforms. The rocks reveal an enthralling chronicle of time that is unfolding and ever-changing. What can the rocks tell us? Think of the colorful layers as pages in a massive book. As the pages are turned, we discover that the words are a language we do not completely understand. The pictures help, but we must put together the story of this ancient book with fragmented clues. The first chapter of this geological text is the Chinle Formation.

Chinle Formation

The colorful badland hills, flat-topped mesas, and sculptured buttes of the Painted Desert are primarily made up of the Chinle Formation, mainly fluvial (river related) deposits. Within Petrified Forest National Park, the layers of the Chinle Formation include the Blue Mesa Member, the Sonsela Member, the Petrified Forest Member, and the Owl Rock Member.

The Blue Mesa Member consists of thick deposits of grey, blue, purple, and green mudstones and minor sandstone beds, the most prominent of which is the Newspaper Rock Sandstone. This unit is best exposed in the Tepees area of the park. The Blue Mesa Member is approximately 220-225 million years old.

The Sonsela Member is divided into three parts: 1) the upper Flattops One Bed, which consists of a thick cliff-forming brown, cross-bedded sandstone, 2) the middle Jim Camp Wash Beds of blue, grey, and purple mudstones and numerous small grey and white sandstone beds, and 3) the lower Rainbow Forest bed consisting of white cross-bedded sandstone and conglomerate of rounded pebbles and cobbles which contains the logs of the Rainbow Forest. This sandstone caps Blue Mesa, Agate Mesa, and the mesa north of the Rainbow Forest. The Sonsela Member is approximately 216 million years old.

The Petrified Forest Member consists of thick sequences of reddish mudstones and brown sandstone layers. This member is exposed in the Flattops and is the white and pink reworked volcaniclastic unit of the Painted Desert. It contains large amounts of petrified wood. The Black Forest Bed, part of the Petrified Forest Member north of Kachina Point, has been dated isotopically at 213 +/- 1.7 million years old.

The Owl Rock Member consists of pinkish-orange mudstones mixed with hard, thin layers of limestone. Lenses of selenite gypsum are scattered periodically throughout the Owl Rock Member representing the minerals left behind after evaporation of inland lakes. This member is exposed on Chinde Mesa at the northernmost border of the park. The Owl Rock Member is approximately 205 million years old.

The colorful layers in the Chinle Formation represent ancient soil horizons. The coloration is due to the presence of various minerals. While the red and green layers generally contain the same amount of iron and manganese, differences in color depend on the position of the groundwater table when the ancient soils were formed. In soils where the water table was high, a reducing environment existed due to a lack of oxygen in the sediments, giving the iron minerals in the soil a greenish or bluish hue. The reddish soils were formed where the water table fluctuated, allowing the iron minerals to oxidize (rust).

Bidahochi Formation

During the Late Miocene and Early Pliocene Epochs of the Neogene Period (4-8 million years ago) a large lake basin with ephemeral lakes covered much of Northeastern Arizona. Fine-grained fluvial and lacustrine (lake related) sediment such as silt, clay, and sand represent the lower part of the Bidahochi Formation. Volcanoes, both nearby and as far as the Southwestern Nevada Volcanic field, spewed ash and lava over the land and into the basin. Many of the volcanoes were phreatomagmatic, when ground or lake-water mingled with eruptive material (magma) to cause explosive eruptions. The resulting ash formed fine-grained deposits that were deposited within the lake sediments.

After a few million years of erosion, most of the Bidahochi Formation has been removed from the park area, leaving volcanic scoria cones and maars (flat-bottom, roughly circular volcanic craters of explosive origin). The vent from one of these maars is exposed on the Painted Desert Rim across the park road to the east of Pintado Point. The Hopi Butte Volcanic Field, which can be seen from the northern overlooks of the park extending northwest, is considered one of the largest concentrations of maar landforms in the world, covering about 965 square miles (2,500 square km). The erosion-resistant lava flows, such as Pilot Rock and the Hopi Buttes, protect the softer lake-bed deposits beneath.

Pleistocene and Holocene Sediments

Pleistocene and Holocene Epoch (1.8 million years ago to present) deposits of windblown sand and alluvium (deposited by flowing water), now cover much of the older formations of the park. At higher elevations in the northern part of the park, 500,000-year-old dunes can be found. Younger dunes, around 10,000 years old, are found in drainage areas that contain sand such as Lithodendron Wash. The youngest dunes are found throughout the park, in all settings, deposited around a thousand years ago. These dune deposits are largely stabilized by vegetation, especially grasses. While not as numerous as the fossils of the Chinle Formation, fossils have been found even in the quaternary sediments, including fragments of an ancestral proboscidean (elephants and their relatives, such as mammoths). The Little Colorado River and its tributaries, including the Puerco River, have cut their own valleys into the soft Chinle and Bidahochi Formations of the Painted Desert.