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Petrified Wood.—Petrified wood is not wood. These logs were living trees; now they are mineral reproductions of those trees. The wood is gone but in its place are jasper, chalcedony, carnelian, and agate—semiprecious gem stones. How this substitution of mineral for wood took place is still something of a mystery. It is believed, however, that the mineral substance known as silica was first carried into the wood by underground waters and there deposited to fill all open cracks, pores, and even the wood cells. Later, the wood tissue of the cell walls was removed and additional silica was deposited in place of it. This substitution of mineral for wood was so perfectly done that not only was the form and size of each log exactly duplicated, but even the minute cell structure of the wood was often perfectly reproduced. The trees grew, were buried, and were petrified during the Triassic geological period.


The Triassic Landscape.—During the Triassic period, which, according to calculations based on the rate of disintegration of radium, ended some 150 million years ago, northern Arizona was a flat lowland close to the level of the sea. Great rivers, flowing from a surrounding fringe of low hills and plains, shifted constantly back and forth spreading layer upon layer of sand, gravel, and muds over this lowland. On the stream banks and flood plains were growing many ferns, giant horsetail rushes (neocalamites), and primitive, coniferous trees (cycads). In the rivers and on the mud flats lived giant crocodilelike reptiles (phytosaurs), large salamanders (stegocephalians), advanced types of reptiles (anomodonts), and primitive lungfish (dipnoi). But, where were the trees that later formed the colorful petrified logs?


The Forests.—Most geologists believe that the growing forests were located upstream, possibly as much as a hundred miles to the west and southwest of the present petrified forests. It is improbable that many of the trees grew within the boundaries of the present monument. The trees were similar to modern pines, but were more closely related to the araucarian pines of South America and Australia. One species (Araucarioxylon arizonicum) predominated, but two other species (Schilderia adamanica and Woodworthia arizonica) are sometimes found in the petrified log jams. No broadleaf trees existed during Triassic times.

There was nothing mysterious in the manner of growth and death of these trees. They grew from seedlings, matured, produced seeds, and died of old age or were killed by disease, insects, fire, wind, or flood in the same manner as modern trees. Doubtless most of the trees decayed away, but some fell into rivers and were carried downstream to their final place of burial on the flat lowlands. In the course of their journey downstream, the trees lost their bark, most of their branches and roots, and even the wood apparently was partly worn away from the outside of the logs. This theory accounts for the occurrence of the petrified logs only in sandstone and conglomerate beds of stream origin, for their concentration in certain restricted areas, and for the predominantly horizontal position of the logs today. It explains why the trees as a rule have no bark, branches, or roots and why it is that among the many perfectly preserved leaves of ferns and cycads there are only rare, fragmental leaves of the petrified trees.

From time to time distant volcanoes gave forth great clouds of volcanic ash which, carried by winds or water, choked the streams, buried the sand and gravel deposits and their included logs, and thus built up a new land surface on which the rivers developed new courses, brought in and buried more logs, and were in turn covered with more volcanic ash.


The Logs Petrfied.—Burial, the first step in petrifaction, was important because it prevented the total decay of the logs. But, more than burial was required, otherwise the logs merely would have turned into carbon or coal. The factor that determined that these logs should petrify was the presence in the ground of mineral waters charged with silica. It is believed that the silica came from the volcanic ash which is found abundantly in adjacent rock strata. The ash decomposed to form the colored, banded Painted Desert rocks, but in the process much silica was freed, leached out, and carried by underground waters into the logs. There, slowly, the silica was deposited, impregnating the wood and replacing the wood fiber until the pine logs became logs of stone.

The Forests Uncovered.—Today, the petrified wood which was formed deep in the earth is on top of the ground. No violent earthquake forced these logs to the surface—they are on top of the ground simply because they have been uncovered by erosion. Blankets of limestone, sandstone, and shale, a thousand feet thick or more, have been stripped away from above the logs by wind and rain. This erosion was accelerated by the gradual uplifting of the Colorado River plateau from sea level to over a mile above the sea. Thus exposed to weathering and erosion, the rocks crumbled, and the rivers carried the fragments away and cut deeper and deeper into the rocks until at last the petrified logs were again exposed. This process is by no means complete, and throughout northern Arizona today there are doubtless thousands of petrified log scattered in the ground, near the surface in some places and as much as three or four thousand feet deep in others.


The Rainbow Colors.—The colors of the petrified wood are not the colors of silica alone. In pure state, silica or chalcedony is white or gray. The great variety of red, brown, and yellow colors are usually produced by minute quantities of iron oxide stain in the silica, and similarly the black is produced by iron and manganese oxide pigment. Sometimes the red and black stain followed cracks in the petrified wood and diffused unevenly into the silica to form the "picture wood" or moss agate.

Crystals of Quartz.—Where wood was present the silica took the form and structure of the wocd, but in open cracks and cavities where there was no wood to interfere, the silica assumed its natural crystalline form—a six-sided prism capped with a six-sided pyramid. Many quartz crystals—red, yellow, green, black, white, clear, and amethyst—thus occur in cavities in the petrified logs.

Semiprecious Stones.—The petrified forests constitute a storehouse of semiprecious gem materials known to the lapidary by a variety of names. The wax-like, colorless quartz is known as chalcedony. The colors of the clear varieties of quartz determine their names, such as carnelian for the red and chrysoprase for the apple-green. The opaque varieties colored red, brown, dark green, or bluish-gray are called jasper. The vari-colored varieties are called onyx if the color bands are straight and agate if the bands are curved or concentric. Moss agate is chalcedony that contains mottled or tree-like designs.

From sources outside the monument private dealers obtain crystals and other forms of quartz to shape and polish into a great variety of ornaments and jewelry settings.


Broken Log Sections.—A most conspicuous feature is the manner in which some of the logs are broken into more or less even sections as if sawed into stovewood lengths. This is an entirely natural phenomenon which, it is believed, was started by earth shocks and pressure while the logs were still buried and tightly enclosed in the rocks. The forces that produced the strain may have been the earth forces that lifted the area from sea level, or the unevenly distributed weight of the rocks above the logs. The original cracks were small, tight, and inconspicuous. However, as the logs are exposed to weathering, these cracks are soon opened and extended until the logs are separated into sections from one to several feet long.

The Painted Desert.—Both the color and the intricate land forms of the Petrified Forest areas and Painted Desert intrigue the spectator. In color, these badlands relate back to the ancient volcanic experience of the region, and in form to the present desert type of erosion.

The material from which the badlands have been sculptured originally was deposited layer upon layer as volcanic ash, probably of drab color. The decomposition of the ash which released silica for petrification converted the ash into the claylike rock bentonite. When pure, the bentonite is nearly white, but in the Painted Desert it is stained all shades of red, orange, maroon, blue, purple, and yellow by iron minerals that also had their source in the volcanic ash.

Bentonitic beds, in arid or semi-arid regions, erode into badlands. The bentonite absorbs water like a sponge, swells, and disintegrates into a fine mud. As a result, the torrential summer rains that fall in northern Arizona rapidly cut the banded, bentonitic beds into sharp, conical hills, turreted ridges, and sharp, interbranching canyons and ravines. When dry, the bentonite is hard and strong and is thus able to preserve these intricate badland forms during the long periods between rains. Locally, a hard sandstone caprock may prevent rapid erosion of the shales beneath to form an abrupt-sided, table-topped butte or mesa.

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Last Updated: 14-Aug-2009