Agatized Rainbows: A Story of the Petrified Forest

by Harold J. Brodrick

Because of the name “Petrified Forest,” many people who have read of it expect to find trees “turned to stone” and standing upright just as they grew. Actually, geologists who have studied the area very carefully do not believe that many of the living trees grew in this particular location, for all of the evidence indicates that fallen timber from forests a considerable distance away was carried here by flood waters of ancient streams and stranded and buried in the mud and shallows of lagoons and marshes.

During the latter part of what geologists call the Triassic period, about 160 to 170 million years ago, most of northeastern Arizona was apparently an extensive flood plain; low, flat, and swampy. Numerous streams, some of them quite large, flowed out from the surrounding low hills into the plain. These streams brought enormous quantities of sediments; mud, sand, and other minerals, spreading it out layer upon layer as they shifted their flow back and forth just as on present river deltas. These sediments contained huge amounts of volcanic ash which the streams apparently picked up near their sources. This ash was largely silica, the mineral which was later to be of major importance in the petrification of the wood. (Silica (SiO₂) is the oxide of silicon, a non-metallic element making up 28 per cent of the earth’s crust. The crystal form of silica is quartz, the commonest of all minerals, which is found in large amounts in many volcanic rocks.)

The flood plain was broken by an occasional ridge or high spot, apparently tree-covered, as a few petrified stumps with partial root systems have been found in the locations where they apparently grew. However, most of the trees grew in forests on the low hills through which these rivers flowed, anywhere from a few miles to 50 or 100 miles or more to the west and southwest of the present national monument. These trees died from various causes, just as trees of our modern times do. Fire, wind, insects, diseases, and other causes all took their toll. Many trees probably decayed in the forest where they fell, but others were picked up by flood waters and were eventually transported by the streams to the flood plain there to become stranded with hundreds of others and to be covered by the sediments brought in by the streams.

This transportation theory is based on several types of evidence. In the first place, the logs have been stripped of much of their original roots and limbs, and practically all of the bark has disappeared. The logs present a worn appearance, an indication of having received rough treatment. Also, very few traces of cones or foliage have been located, although the fossil remains of more than 30 species of fragile ferns, cycads, rushes, and other plants that grew in the marshes of the ancient flood plain have been found. The direction of the original drainage into this area has been established by tracing the source of the Permian gravels which are deposited here.

The deposition of these sediments over the plain continued until a layer about 400 feet thick was built up during the centuries. This deposit is now known geologically as the “Chinle Formation.” One of the principal materials found in the Chinle is Bentonite, originally a volcanic ash which the streams brought. It has since decomposed into a clay-like soil which is very porous and spongy and which readily absorbs water and expands. When becoming very wet, it turns into a bluish mud and is washed away. Erosion of this Bentonite and other materials deposited with it forms the badlands area now seen on portions of the monument and in the Painted Desert. During the ages when the original layers of mud, sand, and silt were being deposited, many of the logs were washed in and buried at various levels with this Chinle material.

While all of this was slowly taking place, the land mass over this part of the continent was gradually subsiding. It continued to settle during the next geological period of millions of years, and layer after layer of sediments were washed in and deposited on top of it. Then during the next geological (Cretaceous) period, a long arm of a sea flooded this part of the country. Marine deposits accumulated on the bottom of the sea until finally the Chinle Formation containing the buried logs was covered by 3,000 feet or more of other deposits.

At the close of Cretaceous time, about 60 million years ago, uplift of the present Rocky Mountain system commenced. The basin in which the Petrified Forest lay buried rose with it. This gradual rising movement has continued intermittently nearly to the present time.

This uplift brought with it the activity of erosion which has continued through the ages until finally almost all of the 3,000 feet of upper layers of material have been washed away, and the many logs, that had once been so deeply buried, have again been exposed on the surface; but now as hard, colorful stone. Erosion continued to carry the soil away from the petrified logs, exposing more and more of them. As forces of erosion lowered the surface of the ground little by little, the petrified logs, too hard to be affected, settled with it, eventually accumulating with sections of other logs that had been buried on a lower level. Thus, the present surface of the ground is rather thickly covered, in many spots, with wood that was originally scattered through approximately the upper 100 feet of this Chinle Formation. In the vicinity of the Rainbow and Third Forest, at least, about 300 feet more of this formation still remains. So far as we know, wood is to be found throughout this entire layer. Therefore, theoretically at least, it may be said that 25 per cent of the petrified wood that is here is visible on the surface, the rest still remaining buried below.

Three species of trees have been found here in petrified form. The most common one is an Araucarian Pine (Araucarioxylon Arizonicum), a primitive member of the pine family. This species became extinct long ago, but there are still several species of modern Araucaria native to South America, Australia, New Zealand, and other South Pacific islands which are apparently very similar to this ancient form. Some of the modern types have been imported to this country and are used for ornamental purposes in certain locations in Florida and along the Pacific coast. The most common ones are known as the “Monkey Puzzle Tree” and “Norfolk Island Pine.” Claims made by roadside stand operators along Highway 66 that the petrified wood offered for sale is “beach walnut,” “cactus,” etc., have no basis in fact.

Two other types of petrified wood are found here in smaller amounts. These are the Woodworthia Arizonica, a cone-bearing tree somewhat similar to the Araucaria and the Schilderia Adamanica, a tree with peculiar radiating rays in the wood. Paleontologists are not sure where this latter kind belongs in systematic plant classification. What happened during the millenniums that the logs lay buried in their Chinle tombs?