The present day Chinle Formation in Arizona is a far cry from the tropics of the Triassic. This paleological site is now defined by the near wasteland of desert, which succumbs to erosion every spring. There are few plants in the hot and dry habitat and fewer animals.  The area is mainly populated with low-lying shrubs and grasses, which are fed on by rabbits and Antelope. Birds, rodents, snakes and other reptiles also share this once rich land but with only nine inches of annual rainfall and over 300 days of hot clear skies per year it is an inhospitable place for any living thing. The colorful, rain cut valleys and gorges attract tourists to its desolate beauty. Hills of every color imaginable, bear of any life but strewn with the remnants of a once lush forest boggle the eyes and mind. Fossilized wood fan be seen where ever you look n every color anyone could ever want. Stones showing detailed inner structures of trees long dead are presented in red, blue, white, black, purple, pink and every variation thereof. Their tissues long since replaced by silicon and other minerals from millions of years buried under the earth. 

There has often been a misconception about the Chinle formation-that it is found in Arizona, in the Petrified Forest National Park. But in reality, the Chinle group consists of two main formations, the Petrified Forest National Park and the Owl Rock formation. The Chinle Formation extends from Wyoming, Utah, Colorado and Arizona. However the expansion of American Territory plays an important role on why Petrified Forest National Park is one of the more famous sites of the Chinle formation.

Early in the age of dinosaurs, 225 million years ago, Arizona was a tropical lowland and was located some seventeen miles closer to the equator. Due to the yearly erosion of land, trees and logs were washed down from the highlands and over the years became mired in mud and covered with sediment. Like trees and logs, animal remains were fossilized and exposed by erosion centuries later. Most of the wood and bones in Petrified Forest (Arizona) were in fact petrified, that is, turned to stone by a process in which practically all of the organic matter is replaced by minerals.

Late in the Triassic period, Pangea began to break up. Starting in the part of Pangea that is today western Europe, crustal extension caused rifting. This rifting eventually lead to the separation of North and South America. This movement continued on, eventually leading to the separation of Pangea into many of the continents we know today. With the rifting of the continents in the break up of Pangea, great ravines and rifts in the ground formed, and after streams shifted into them, great lakes formed.

The beginning and end of the Triassic period were marked by crises which caused great extinctions. The first lead to the extinction of several corals and many echinoderms. This opened up territories, and lead to the development of modern choral groups, as well as tachyosaurs, among other things. The second crisis led to the extinction of phytosaurs and placodonts.

The Chinle formation was marked by two distinct periods during the Triassic period. The early Triassic period was extremely wet at the Chinle formation. This can be seen by fossils of animals which loved the wet environment. Including metoposaurids ("large-headed" amphibians") and long-snouted phytosaurs. A large amount of shale is present in the lower layers of the Chinle formation that made up the petrified forest.  This points to the wetness in the early part of the Triassic at the Chinle formation, because when mud is pressed with great pressure for long periods of time, it becomes shale.

The second period of the Triassic was much more arid at the Chinle formation. There were large flood plains, and streams meandered throughout the site. There was a huge lake in the center of the site (which led to the Owl Rock members of the Chinle formation.) Perhaps this lake was created by one of the rifts formed during the break-up of Pangea.

The flora of the Triassic consisted mostly of ferns, conifers, cycads, but none of the flowering plants we see today. Through the Triassic period, there were two major groups of land animals. The first appeared during the early Triassic period, and were the synapsids. These were the early precursors to the mammals. The second large group of animals were the Archosauromorphs, which included crocodiles and pterosaurs, and during the late Triassic the dinosaurs.

Most of the petrified logs and stumps are Araucarioxylon arizonicum and are related to the modern Norfolk pine and Monkey Puzzle tree.  The ancient Araucarioxylon forest very likely resembled modern redwood forests along the Pacific Coast.  Two additional types of silicified trees were Woodworthia arizonica and Schilderia adamanica.  Schilderia were small, growing to heights of only twenty to thirty feet while Woodworthia might grown up to fifty feet tall with trunks three feet in diameter.

Other identified plants are horsetails that commonly occurred in the Chinle at a number of places.  Ferns also were abundant throughout the Chinle, as were cycads.  Scientists have found abundant leaves from a similar plant, Bennettitales, an extinct group that may be related to the flowering plants.  Other plants have proven difficult to classify; one of these is designated Dinophyton spinosus-the "terrible spiny plant." All of the plants that flourished in Petrified Forest in the Upper Triassic are extinct, but modern descendants of some of them, such as ferns and conifers, live in humid, tropical parts of the world.  Most of the major plant groups, except flowering plants, which appeared on earth later, are represented in the Chinle Formation.

The Chinle Petrified Forest, located in Arizona, contains fossils of various interesting organisms. Among the fossils, animals of these three categories: fish, Tetrapods and invertebrates can be found. Among the fossilized fish were the carnivores Chinlea and Turseodus, which fed on invertebrates and other fish. Also present was the omnivore Ceratodus, which  fed on mollusks, soft-bodied invertebrates and plants.  There were also the bottom  feeders (Cionichthys) which fed on organic debris and small bottom-dwelling organisms.  Last but not least, the Hemicalypterus nibbled on  sessile and floating organisms. One may see from the differences in their  diets, that different kinds of fish were able to inhabit the same environment, having to claim different  niches. 

Tetrapods, both aquatic and terrestrial were also evident. Aquatic carnivores include the Metoposaurus, an amphibian that fed on fish, and the phytosaur, Rudiodon. The Rutiodon dominated the Chinle food pyramid, eating anything it caught. On land were the carnivores Hesperosuchus, a pseudosudrian, fed on small reptiles and insects, and the Coelophysis which fed on reptiles. There was also, of course, the herbivores  which fed on the vegetation present such as fibrous plants and tubers. The Desmatosuchus  and the Typothorax, both dicynodonts, belong in this category.  Though not as large and powerful as the other organisms described above, there were invertibrates such as the worms, snails, limgets, etc.  From the fossils we see that there was an abundance of Pelecypoda, members of the family Uniodae, in the fresh water portion of the Petrified Forest.  There were also various Gasropodas including Valvata gregorii, Lymnaea  hopii, Triasamnieola assiminoide and many more, which were first  discovered in 1951 at the three localities in the Owl Rock member of the  Chinle formation. From the fossil records found in the Chinle formation, we  see that Chinle was a habitat for many many species.

The taphonomy of these species concerns the preservation or how these species are fossilized. Most vegetation like fallen leaves in the Chinle formation were preserved in amber just as the process seen in the popular movie Jurassic park. However, the fallen logs found in the Petrified Forest National Park were preserved due to the materials present in the mud and silts which replaced the minerals and other compounds found in a species.

The position of the fossils when found determine how fast a species was fossilized. If the bone structure of an animal is well intact, it often suggests that the animal was instantaneously preserved. Most fossils found in the Chinle formation are red in color due to the silt present in the Triassic period. However, because fossils, especially that of plants are preserved in thin carbonaceous compounds, the cuticle of the plants break when paleontologists try to recover them.  There are often deposits of dirt and mud on the fossils. Attempts to remove these stubborn deposits, by physical methods, such as grinding or using air abrasive tools, most often results in permanent damage to the specimens regardless of their composition. These "hematitic matrices (deposits on fossils)" can be removed by dipping the fossils in aqueous solutions of thyoglycollic acid. Calcium orthophosphate is also added to the acid solution in order to prevent the acid from removing the calcium in the bone. Chemical use is better and faster than the physical process.

The Chinle Formation in Arizona has been inhabited for over two thousand years.  The first inhabitants were ancestors of today's American Indians.  Known by the Navajo as Anasazi, meaning "enemy ancestors" or "ancient ones", these primitive people were hunter- gatherers.  Living in caves from 100 BC until around the beginning of the eighth century, they eventually began building pueblos out of the petrified wood found in the area.  These pueblos sometimes contained more than one hundred rooms and would house nearly eighty people.  By the eleventh to thirteenth centuries the Anasazi were living in large stone villages with some buildings standing several stories tall.  The Anasazi used the petrified wood for more than just construction though.  Arrowheads and axes were made from the stone until the early 19th century when the area was taken over by the U.S. government.  But because of droughts and a loss of areas for growing crops the area was abandoned by the early 1300's.  The Spanish invasion in 1540 marks the beginning of the area's recorded history, although no mention of petrified wood has been found in Spanish documents from that time.  

The first account of petrified wood in the American southwest was made on September 5, 1849 by Lt. James H. Simpson of the U.S. Army Corps of Engineers.  Lt. Simpson was a member of an expedition party, mapping the newly acquired land, who camped near Canyon de Chelly near the Shinarump Member.  His report was very brief noting only that a part of a tree was exposed from the side of the canyon and that it was petrified.  The man most usually credited as the discoverer of petrified wood in the American southwest was Capt. Lorenzo Sitgreaves.  On September 28, 1851 Sitgreaves and his U.S. Army expedition team found a large deposit of highly colored petrified wood.  The exact location isn't known but it was probably a few miles south of the modern Petrified Forest National Park in eastern Arizona near the junction of Milky Wash and the Little Colorado River.   

Probably the most important discovery at the time concerning the Petrified Forest, as it was beginning to be known, was made on December 2, 1853. Another large deposit of petrified wood was discovered by the Army Corps of Engineers under the command of Lt. Amiel W. Whipple.  After surveying the area he named it Lithodendron Creek, a name that would later be modified to Lithodendron Wash.  This area, now a part of the northern Black Forest section of Petrified Forest National Park is characterized by the dark, almost burnt appearance of the petrified wood.  This was the first expedition into the area to include a professional geologist.  The French- Swiss geologist, Jules Marcou, concluded that all of the wood present was coniferous and thought that it closely resembled formations he had seen in Germany which dated to the Triassic era.  Marcou took extensive notes and sent several specimens back to Europe but fell ill and was forced to return to Europe before a complete report could be finished.  His notes were later handed over to William P. Blake of the U.S. Pacific Railroad Surveys and expeditions.  Blake disagreed with Marcou's description of the wood's age and reported that he believed it was probably from the Carboniferous era.  This was the first official report on the age of the wood specimens.  

Another important member of the Whipple expedition was a German artist named Baldwin Mollhausen.  In his journal he included detailed descriptions of the area and in 1885 published a book in both German and English editions.  This became the most thorough and descriptive account of the petrified wood found in the southwestern United States.  Four years later yet another Army expedition was led through the region and yielded yet more reports and revelations.  On May 7, 1858 an expedition led by Lt. Joseph C. Ives discovered more petrified wood 60 miles south of Lithodendron Wash.  The expeditions' physician, Dr. John S. Newberry, examined many of the specimens in the area and concluded, "...all had been transported, but not far from their place of growth." (Ash, p. 51).  This revelation was an important discovery given the fossils that were later to be found in the area.  He concluded, because of the fact that nearly every trunk found was shorn of all branches, leaves and bark, that the specimens in the area had grown in nearby forests and hills.  During seasonal rains, floods had dragged these trees downstream and deposited them in either a lake or muddy flood basin, where they were quickly covered and eventually became fossilized.  Although no fossilized animal remains had been found at the time, his assumptions were legitimized some years later when a number of fossils were found of apparently semi-aquatic creatures.  A year later on a second expedition into the area, Dr. Newberry discovered the first fossilized leaf remains in a sandstone copper mine in the Arroyo del Cobre in northwest New Mexico near the Arizona border.

In 1878 General William T. Sherman requested that Lt. P.T. Swain, based at a nearby fort, obtain two logs from the Lithodendron Wash to be sent to Washington D.C. for display in the Smithsonian Institute.  Both specimens are still in their collection although one of the logs was actually found several miles north of the wash.  Up to this time no animal fossils had been reported but in 1901 a geologist named Barnum Brown was working in the area near Cameron, Arizona when he discovered the first Metoposaurus fraasi remains, a large amphibian that would eventually be found in large numbers within the National Park.  Around this time he also discovered the first Placerias hesternus.  Placerias was a rhinoceros-sized herbivore with a small bony frill covering the neck and two short tusk-like horns protruding from the corners of its mouth.  This was the first evidence of a North American synapsid called a dicynodont.  Brown worked within the Park for nearly 50 years, discovering many fossils during that time.  Throughout the 1920's and early 1930's archeologist, Charles L. Camp worked all over the Chinle Formation and collected numerous fossilized remains.  In 1930 Camp wrote the first definitive book on phytosaurs, crocodile like reptiles that once dominated the region.  He listed six species within the Chinle Formation and presented them in a chronological order of evolution, which has been regarded as essentially correct to present times.  Several collections of remains from this area are currently in the possession of the University of California, Berkeley, including the collections of John Muir from near Adamana in 1905-06 and Sidney Ash's plant collection. Ms. Annie Alexander and Louise Kellogg also sent collections back to the university in 1921 when they made a trip to the park for excavation.

By this time the Petrified Forest had grown in popularity among Arizona's still small tourist base.  Given the fact that the Petrified Forest was only eight miles south of the Adamana station on the Santa Fe railroad line, many souvenir collectors began collecting large amounts of petrified wood.  As well as tourists, some of the wood had been shown to jewelers and many of them ordered large pieces of wood shipped across the United States for use in jewelry, floor tiles and table tops.  These pieces of petrified wood came at a high price and many companies soon leapt up to satisfy the demand.  However, soon after this short boom many of the craftsmen discovered that the wood was both extremely hard and very brittle, many times cracking before the artist could finish his work.  Because of the hardness of the petrified wood another entrepreneur of the time discovered that the material would create a good abrasive if crushed.  To this end a mill was installed in the town of Adamana during the early 1890's to crush the wood.  This was a wake up call to the nearby inhabitants who, in 1895 petitioned Congress to have all areas containing petrified wood withdrawn from public use until examined as a possible national park.  In 1899 a government hired geologist, Lester F. Ward, was sent out to examine the area.  After several weeks in the area he sent a report back to Washington in 1900 recommending that the area known as Chalcedony Park be set aside for government protection.  On December 8, 1906 President Theodore Roosevelt took a trip to Arizona and declared the area a National Monument.  Eventually the area was enlarged to include Lithodendron Wash, and on December 8, 1962 the area was declared a National Park.