Volume 2 - No. 4
By M. V. Walker,
The word fossil, as applied to fossil forests, may be defined as "any indication of the former existence of plant or forest growth." These indications may be any of the following: petrified wood, coal, fossil leaf impressions in sandstone or shale rocks, casts or molds of trunks, branches, leaves, and sometimes cones and fruits. A fossil forest might be made up solely of the very delicate leaf impressions of the trees and shrubs of some prehistoric forest, with these impressions now preserved in the sandstones and shales of various geological periods.
The term petrified forest infers a "change to stone", so that we should expect to find a forest literally turned to stone. This as indeed far from the truth, for there are certain very definite limitations regarding the possibility of an object being petrified. To petrify, the specimen must be solid like wood, yet porous and cellular enough for the infiltration of water in which various minerals have been dissolved. For that reason a petrified forest is bound te be limited to the trunks and branches of the trees and shrubs which made up the original living forest. A petrified forest is also a fossil forest, but many of the fossil forests are not petrified forests.
The Petrified Forest National Monument, Arizona, is the most outstanding petrified forest in the United States. It is also a fossil forest, for, in addition to petrified logs and branches of a prehistoric forest, it has fossil evidence of the leaves of the trees, the cone scales, and the undergrowth which was composed of various types of ferns, rushes, and cycads.
In north central Kansas there is an exposed layer of sandstone called the Dakota Sandstone. Very few fragments of petrified wood are to be found in this formation but some of the sandstone has many beautiful impressions of fossil leaves which indicate the former presence of a great forest. This is a fossil forest where scientists recognize such forms as eucalyptus, sassafras, birch, beech, walnut, and acacia. These are forms that for the most part are now found far away from the plains of Kansas, yet the forest grew and flourished there many millions of years ago.
Although each one of our petrified forests may have a slightly different story as regards the type of mineral repleacement, the geological age, or the kind of trees, a number of general conditions were the same in each instance. The tree trunk or branch did not remain on the surface of the ground or the water, or it would have decayed. The first step in its preservation was that it was either buried in the ground or submerged in water, thereby protecting it from the elements of wood decay and disintegration. A log would sink in the water when it became water-logged, or it could be buried in the ground by the deposition of silt or sand carried by streams during flood periods. Volcanic eruptions of ashes and mud might cover such objects as were on the surface of the ground.
HOW OLD ARE THE FOSSIL FORESTS?
The second step requires that water, which has in solution certain minerals, must come in contact with the buried wood and completely saturate it. This may be the normal ground water or mineral charged water of lake or stream.
The third step, and the one which is perhaps least understood, involves the deposition of mineral matter in the cells and woody portion of the tree. Undoubtedly this infiltration and precipitation must be repeated many times before a large tree trunk can be completely replaced with minerals and thus petrified. In some specimens this replacement was so slow and perfect that the original structure of the cells, growth rings, and pith rays are easily recognized. In others the conditions were such that although the outward appearance of the tree trunk is unmistakable, the entire inner structure is a mass of mineral material without a trace of the original cellular and woody structure. This last example is hardly more than a cast or mold.
The fourth step has to do within the presence of the petrified wood on the surface of the ground today. The answer is that although the trees were once buried perhaps by hundreds or thousands of feet of rock, erosion has since removed this covering and once more these objects have come to rest on the surface of the ground.
Thus we see that the recipe for a petrified forest requires burial or submergence; infiltration and saturation; mineral deposition (replacement or infilling) by precipitation; and finally, exposure by erosion. Like all good recipes there are possibilities of unlimited slight deviations, each one producing peculiar results which only add to that long list of nature's secrets.
"How long did it take?" is one of the interesting questions about petrified forests. The answer is far from satisfying, for very little is known about the time involved in the actual mineral replacement. Modern experiments have proven that it could be quite rapid, or again the replacement of even a small branch might require hundreds or thousands of years. It seems that the process can be speeded up, slowed down, or brought to a complete standstill by slight variations in mineral content of the water; by introduction of foreign minerals, or by failure to provide proper conditions of saturation and deposition of the minerals carried by the water. The placing of small blocks of wood in certain of the hot springs of Yellowstone National Park, Wyoming, and the study of others which were found in the springs, give us a clue to the process of replacement or infilling, and also give ample proof that the time element involved in the replacement is an extremely variable one.
Petrified wood may occur in various forms. Since the replacement process involves a deposition or concentration of the minerals carried by water, a great variety of minerals might be in solution in the water which would result in various types of petrifaction or replacement. If calcium carbonate (lime) is in solution in the water and replaces the wood, we have what is called calcification. Where silica is the replacement mineral, we call this silicification. When the water carries iron and sulphur, and the replacement mineral formed is pyrite, we have what is called pyritization. Where slow decay under water results in the concentration of carbon, the fossil log may consist of carbon, showing more or less distinctly the form and structure of the original tissues. This is called carbonization.
Silicified wood is extremely hard and durable. All of it does not look the same, for the presence of various impurities in the water along with the dissolved silica, such as iron, manganese, and aluminum, may produce not only different forms of structural replacement but will often result in the formation of beautiful colors and tints. Thus when the silicified wood is highly colored we may, depending in each case on the color, get such forms of petrified wood as agatized, opalized, jasperized, or carnelianized. This explains the beautiful agatized wood in the Petrified Forest National Monument, and the highly colored opalized wood of Nevada, Montana, and Washington.
To give some idea of the widespread occurrence of fossil and petrified forests, attention is called to the following localities which have been selected mainly because they represent a fairly wide variety of types and conditions:
Petrified Forest National Monument, near Holbrook, Arizona - This is the most outstanding of all the areas, for, although it is called a petrified forest, it is one of the most complete fossil forests of the world. Originally established as a national monument to protect the great number of beautifully colored agatized and silicified tree trunks, it is now becoming known for its deposits of leaf and plant impressions of not only the trees, but also of all the plant forms that were present when the forest grew, such as ferns, cycads, and neocalamites. The petrified bones of great reptiles and amphibians that lived around the lakes and pools of this once luxuriant forest are also found here. It is believed by some authorities that some of the trees were buried upright in the position in which they grew. Most of them are on the ground. This may indicate that they were buried where they fell. However, this theory is still a matter of controversy. Others were carried by rivers and streams for some distance, as shown by the battered and torn ends.
Gingko Fossil Forest State Park, near Ellensburg, Washington, on the Columbia River - This area contains among its petrified tree trunks the remains of a very interesting type of tree called the Gingko. It is commonly known as the Maiden-hair tree. Although once widely distributed over the world it is now native only in China end Japan. An old lava flow covered the sedimentary rock in which this forest probably grew. The petrified logs are buried in the lava, which is believed to have flowed into a lake basin in which there were either water-killed stumps, water-logged trees, or floating driftwood. As the lava cooled, a deposit known as "pillow lava" was formed around the tree remains. The replacement mineral is silica, in a form commonly called opal.
Florissant Fossil Forest, near Colorado Springs, Colorado - Some of the petrified. trees have been identified as sequoias, thus proving that the sequoia trees were once much more widely distributed over the United States than they are today. The deposit, which seems to be an accumulation of silt and volcanic mud and ash in an old lake basin, is also noted the world over for its fossil remains of insects, fishes and other animal forms. Many of these are preserved as very delicate impressions in the fine-grained thin-bedded shales of this miocene lake deposit.
Redwood or Calistoga Petrified Forest, near Calistoga, Sonoma County, California - Here we have the petrified remains of great sequoia trees that we reburied and preserved by the volcanic eruptions of Mt. Saint Helena. Some of these specimens are very large and many appear to have fallen in the same direction, that is, away from the onrushing volcanic deposits. Continued excavating would probably reveal a large number and variety of trees.
New York Fossil Forest, on Schoharie Creek, near Bilboa, New York - This is often referred to as "the oldest known petrified forest", for the large stumps of tree ferns date back to the devonian period, some 337,000,000 years ago. Other tree-like forms here seem to indicate that in this period the first true forest growths appeared. Reconstructions and restorations of this devonian forest are on exhibit in the New York State Museum in Albany.
Fossil Forests of Yellowstone National Park, Wyoming, near the Lamar River - Although there are several fossil-bearing localities in Yellowstone National Park, the petrified forest areas are probably the most widely known. Thnis petrified forest is known the world over because erosion has now exposed not only one, but several standing petrified forests, one above the other, indicating that a forest grew and was buried and another grew and was buried, until some twelve or more successive forests had been buried by the volcanic ashes and mud that accumulated and finally filled this old basin.
Fossil Cycad National Monument, near Edgemont, South Dakota - Petrified trunks of cycads, another type of tree now extinct in North America, were found so abundantly in a small area here that it was set aside as a national monument. The replacement of these specimens was so perfect in every detail that when thin sections were made and studied microscopically, it was possible to reconstruct the actual flower parts with great accuracy. The cycads, like the gingkos, became extinct in North America for no explainable reason, and are today found native in only a few isolated places in the world.
Black Hills Fossil Forests, near Rapid City, South Dakota - There is a large amount of petrified wood scattered throughout the jurassic and cretaceous deposits exposed around the Black Hills region. Many of the specimens are well preserved silicified trunks of trees that measure several feet in diameter and occasionally 100 feet long.
Texas Fossil Forests, near Glen Rose, Madera Springs, Big Bend National Park Project, and Palo Duro Canyon State Park - Petrified wood is widely scattered over Texas, and in some instances it is highly colored. In some sections, such as near Glen Rose and Fort Worth, these forests have been almost completely destroyed by private individuals who have taken the petrified wood for use in constructing buildings and fences. Most of these petrified and fossil forests are found in exposures of the cretaceous age. Early cretaceous forests have been pretty well reconstructed from studies of the fossil leaves found in such exposures in several Texas localities. Petrified wood of triassic age is quite abundant in the Palo Duro Canyon State Park, near Amarillo.
Utah Fossil Forests, near Zion National Park, and Capitol Reef National Monument - Petrified wood is found in many of the triassic and cretaceous exposures in this region. Large silicified logs several feet in diameter and 50 or more feet in length may be seen in the southwestern corner of Zion National Park, and also near the village of Fruita, in Capitol Reef National Monument. There are a few fine petrified logs in the triassic and cretaceous exposures east of Bryce Canyon National Park and scattered over the Escalante and Paria drainages. Fossil loaf impressions of ferns, cycads, and neocalamites have been found in the chinle exposures in Zion and Capitol Reef, and cretaceous fossil leaves have been found in the sandstones below Bryce Canyon National Park. A silicified trunk of a cycad was found as a washed boulder along the Virgin River in Zion Canyon.
The study of fossil forests has made possible the reconstruction of typical scenes of each of these different geological periods. The student of historical geology has been able by a careful study of these remains to visualize the conditions that prevailed in each area. Research in one area has made it possible to reconstruct the trees with their leaves, foliage, and in some cases, their flowers. In an other instance, through indirect evidence, there appear signs of attacks by insects such as bark beetles, thus making it possible to picture the forest pests of bygone days. The underbrush of some fossil forests may even be worked out in detail, one area having growth of ferns, rushes, and cycads, while in another the growth was composed of shrubby currants, laurels, dogwood, and vines. The animal life that was present in the old forests may sometimes be pictured when the fossil hunter is fortunate enough to find the petrified bones, teeth, and skeletons of giant reptiles and amphibians. In other regions the delicate impressions in the shale reveal such forms as bees, butterflies, beetles, flies, and wasps.
One of the interesting facts learned from the study of fossil forests is that many types of trees which are now restricted in their natural range, such as the sequoia in California, were once widely distributed over the United States. For example, there ere fossil sequoias in Colorado and Wyoming. Such forms as gingkos, cycads, and araucarias, now found only in very restricted growths in foreign countries, once grew luxuriantly in Washington, South Dakota, and Arizona. The dense jungles of ferns, rushes, and cycads indicated by the fossil remains in ethe Petrified Forest National Monument would tend te prove that this region which is now an arid plateau country, was once a swampy lowland with an abundance of moisture. A study of fossil forests is a key to the past in which is revealed a wide variety of forest growths in as many different environments, some tending to indicate that temperate or semi-tropical humid conditions once prevailed in areas that today are arid and frigid; or that low valleys and inland lakes once existed where we now have high mountain ranges.
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