Fossil Forests of the Yellowstone National Park
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The manner in which these forests were fossilized may next be considered. Though the whole history of the process is not fully understood, it was undoubtedly dependent on or at least greatly facilitated by the presence of volcanic and hydrothermal activity, which was doubtless then, as it is to some extent now a marked feature of the park region. At least a hint of the probable process is afforded by the action now going on in the hot spring areas. Many of those areas are closely surrounded by forests, and unless the action of the springs is very violent the trees may be growing only a short distance away. Occasionally a hot spring may break out near the edge of a forest, the first effect being, of course, to kill the trees. In a few years, by the action of the ordinary processes of decay, a tree so killed may have lost its bark and most of its smaller branches. The hot water which constantly or intermittently surrounds the tree contains a considerable amount of silica in solution, and as this hot silica-charged water is drawn up into the wood by capillarity the silica may be deposited in the cells of the wood after the water cools or evaporates. The first result will be a more or less complete cast of the interior of the cells and vessels of the wood. This much of the process has actually been observed, but as decay is more rapid than silicification, the wood crumbles to dust before petrifaction is complete. If the trunk could be surrounded by ashes or mud and thus protected from atmospheric action, it might in time be completely turned to stone.

The fossil forests are surrounded by a matrix that is known as an acidic lava—that is, a siliceous lava—which contains abundant silica in solution. The first part of the process of silicification may well have been that above described as taking place in the hot spring areas at the present day—that is, the silica would be deposited in all the cells and vessels of the wood, making an accurate cast of all open spaces. Then, while the slow process of decay went on, as each particle of organic matter was removed its place was taken by the silica, until, finally, all the wood substance had disappeared and its place atom by atom had been taken by silica.

By this or a similar process the wood has been preserved or fossilized with remarkable fidelity; in fact, thin sections or slices of the fossil wood may be studied under the higher powers of the microscope with almost or quite as much completeness and satisfaction as if they were sections cut from a piece of living wood. Each cell and vessel, with its characteristic pits and markings, is preserved exactly as it grew. Some of the wood, however, was evidently more or less decayed before it was fossilized, or else decay worked faster than replacement, so that in some fragments the structure is not so clearly preserved. Many of the trunks were subjected to pressure before replacement was complete, and this has crushed or distorted the cells. On the whole, however, the wood is exceptionally well preserved, as may be seen in figures 12, 13, 14, and 15. These are all magnified 100 diameters and were photographed directly from the thin sections—that is, they are photomicrographs—and have not been retouched in any manner. Figure 12 shows a transverse section of the wood of the large redwood trunk that has been so often mentioned (see title page). The section is cut through one of the growth rings, which consists of 12 or 15 rows of very thick-walled cells. The large, regular thin-walled cells, which begin abruptly above the growth ring, belong to the spring wood—that is, the wood first formed after growth starts in spring, when the supply of nourishment is abundant. If there is sufficient moisture and all conditions are favorable this vigorous growth of wood cells may continue without interruption until the approach of cold or dry weather, but not infrequently there may be a brief shortage of moisture, and this is reflected in the formation of a few rows of thicker-walled cells. Such a condition may be observed in the present specimen, in which a slight, partial ring may be seen at some distance above the main ring.


The very perfect preservation of the wood of one of the pines (Pityoxylon amethystinum) is shown in figure 13, a view of a section cut through a part of a growth ring and into the spring and summer wood, the rings in this species being so broad that it is impossible to show a complete one. The opening near the bottom of the figure shows one of the large resin ducts, which, in the living wood, is filled with the "pitch" that so readily exudes when a branch is cut or broken. A longitudinal section of the other species of pine (Pityoxylon aldersoni) is shown in figure 14. The many little rows of superimposed cells in the midst of the long wood cells are the cut off ends of what are known as medullary rays—that is, the little plates of cells that connect pith and bark. One of the resin cells cut in the long direction is shown near the center of the figure; the contents are much darker than that of the wood cells.



The very great difference between the sections of coniferous wood just described and the wood of a deciduous tree is brought out in figure 15, which is a longitudinal section of a laurel (Laurinoxylon pulchrum). In this the wood cells are relatively much smaller and shorter, and the medullary rays are in several irregular rows. The large dotted duct near the middle of the figure is a feature not present in coniferous trees.



An enumeration of the kinds of trees that are represented by the woods in the fossil forests of the Yellowstone National Park will naturally be demanded. A superficial or macroscopic examination of these trunks would not permit a close decision as to the kind of wood; in fact, it would hardly be possible to do more than separate them by this means into coniferous and dicotyledonous trees. But by studying thin sections under the microscope it is possible to distinguish the different kinds with reasonable accuracy. As the result of such study the following species have been detected:

Magnificent redwood (Sequoia magnifica).
Alderson's pine (Pityoxylon aldersoni).
Amethyst pine (Pityoxylon amethystinum)
Laurel (Laurinoxylon pulchrum).
Aromatic bay (Perseoxylon aromaticum).
Hayden's sycamore (Plantaninium haydeni).
Knowlton's sycamore (Plantaninium knowltoni).
Felix's buckthorn (Rhamnacinium radiatum).
Lamar oak (Quercinium lamarense).
Knowlton's oak (Quercinium knowltoni).

Although only three kinds of coniferous trees have thus far been found in the fossil forests of the park, fully 95 per cent of all the trunks belong to these three species. The preponderance of conifers is probably due to the facts that they were presumably more abundant in the beginning, and that, in general, coniferous wood decays less rapidly than that of most of deciduous-leaved trees. But the conditions were so favorable for preserving any wood that it is perhaps strange that not more trunks of deciduous-leaved trees have been found there. As it is, however, a greater number are known from the park than from any other region. Thus, the Arizona fossil forests embraced only two species of deciduous-leaved trees; the Calistoga (California) wood only one species, and the forest at Cairo, Egypt, only four species.

The 10 species of trees represented in the fossil forests of the park are by no means the only fossil plants that have been found. The fine grained ashes and volcanic mud in which the forests were entombed contain also great numbers of impressions of plants, many of them very perfectly preserved. Most of these are impressions of foliage, such as fronds and leaves, but they include also roots, stems, branches, fruiting organs, and even what is believed to be the petals of a large magnolia flower. About 150 different kinds of fossil plants have been found in the park, 80 in the same beds with the forests, and most of the others in slightly higher and younger beds. The list embraces 10 ferns, among them a fine chain fern (Woodwardia), several aspleniums, and a beautiful little climbing fern (Lygodium). The horse-tails (Equisetum) are represented by 4 species. The conifers include no less than 6 species of pines (Pinus), a yew (Taxodium), and 2 sequoias. These have been identified either from the foliage or the cones, and it is more than likely that some of the specimens may represent organs that belonged to trees represented by the fossil trunks, but as they have never been found connected they have been described separately. The monocotyledons, or plants with parallel-veined leaves, are represented by only a few forms, such as a single large grass (Phragmites), a few sedges (Cyperacites), a smilax, and a curious broad-leaved banana-like plant (Musophyllum). The dicotyledons, or deciduous-leaved plants, make up the bulk of the flora and include walnuts (Juglans), hickory nuts (Hicoria), bay berries (Myrica), poplars (Populus), willows (Salix), birches (Betula), hazel nuts (Corylus), beech nuts (Fagus), chestnuts (Castanea), oaks (Quercus), elms (Ulmus), figs (Ficus), breadfruits (Artocarpus), magnolias (Magnolia), laurels (Laurus), bays (Persea), cinnamons (Cinnamomum), sycamores (Plantanus), acacias (Acacia), sumachs (Rhus), bittersweet (Celastrus), maples (Acer), soap berries (Sapindus), buckthorns (Rhammus), grapes (Cissus), basswood (Tilia), aralias (Aralia), dogwoods (Cornus), persimmons (Diospyros), ash (Fraxinus), and a number of others without vernacular names.


A brief comparison of the fossil forests with the forests now living in the Yellowstone National Park may be of some interest. The present forests are prevailingly coniferous, the most abundant and widely distributed tree being the lodgepole pine (Pinus murrayana), which forms dense forests over much of the plateau region. It is distinguished by having the leaves in clusters of two. It is a tree with a slender trunk, usually 70 or 80 feet high, though in exceptionally favorable localities it may reach a height of 150 feet. Its diameter rarely exceeds 2 or 3 feet. The areas ravaged by forest fires are usually reforested by this pine alone, and the young trees come up so close together as to form thickets that can scarcely be penetrated.

There are two other pines in the park, both white pines, allied to the common white pine of the Eastern States, and like it both have the leaves in clusters of 5. One, known as the Rocky Mountain white pine (Pinus flexilis) is a small tree, only 40 or 50 feet in height, and usually grows singly or in small groves. The other, called the Western white pine (Pinus albicaulis), is still smaller, being usually 20 to 30 feet high, and has a short trunk some 2 to 4 feet in diameter. It grows on high slopes and exposed ridges.

Perhaps next in abundance to the lodgepole pine is the white or Engelmann spruce (Picea engelmanni), a tall, handsome tree with disagreeable smelling foliage. Another rather abundant tree is the Douglas spruce, or red fir (Pseudotsuga mucronata), which, where best developed on the Pacific coast, attains a height of 200 feet, though in the drier interior it is rarely over 80 or 100 feet high. There are also two species of fir, the white fir (Abies grandis) and the Balsam fir (Abies lasiocarpa), and a single juniper (Juniperus communis siberica), which is often scarcely more than a shrub.

The deciduous-leaved trees are almost a negligible element in the present park flora, being confined to an occasional cottonwood (Populus angustifolia) at the lower elevations, along the Yellowstone River, and small groves of the quaking aspen (Populus tremuloides). Along the streams and in wet places there are many species of willow (Salix) and several alders (Alnus), and in mountain bogs and valleys there is a small birch (Betula glandulosa). There are, of course, many small shrubs, such as gooseberries, currants, and roses.


The question is often asked, HOW old are the fossil forests? It is, of course, impossible to fix their age exactly in years, though it is easy enough to place them in the geologic time scale. The stratified rocks that make up the crust of the earth, from the oldest we know to the most recent, have been divided by geologists into a number of major divisions or systems, each—except perhaps the oldest-containing the remains of certain kinds of plants and animals. The accompanying diagram (fig. 16), shows these major time divisions, arranged in their proper sequence from the lowest to the highest. The star (*) in this geologic time scale indicates the age of the rocks in which the fossil forests were entombed. It shows that they were buried during the Tertiary period. This period is divided into four epochs, the oldest called Eocene, having been succeeded in turn by the Oligocene, the Miocene, and the Pliocene, which just precedes the Pleistocene or glacial epoch. The forests of the Yellowstone National Park are found in the Miocene series of the Tertiary. As compared with the eons of geologic time that preceded it the Miocene is relatively very recent, though, if the various estimates of the age of the earth that have been made by geologists are anywhere near correct it may well have been a million years ago. It must be remembered, however, that this estimate involves more or less speculation based on a number of factors which may or may not have been correctly interpreted.

Pleistocene (glacial).

A study of the fossil trees themselves gives at least a rough approximation as to the length of time it may have taken to accumulate the beds in which they are now buried. As already mentioned, there is a succession of forests, one above another, through a thickness of 2,000 feet of strata. The unit of the measure of the time is the time taken by each forest to grow. Pine trees of the types represented in the fossil trunks require 200 or 300 years to reach maturity, and redwoods may require from 500 to 1,000 years. Twelve or more of these forest levels have been found. By multiplying this number by the minimum age of the trees (200 years) we shall have 2,400 years, and by multiplying it by the maximum age of the redwood (1,000 years) we shall have 12,000 years as the possible time during which these forests flourished. It is possible that the truth lies somewhere between these extremes.


A final word may be added regarding the probable climate of the region during the lifetime of these fossil forests. It is obvious that the present flora of the Yellowstone National Park has comparatively little relation to the Tertiary flora and can not be considered the descendant of it. It is also clear that the climatic conditions must have greatly changed since Tertiary time. The Tertiary flora appears to have come from the south, but the present flora is evidently of more northern origin. The climate during Tertiary time, as indicated by the vegetation, was temperate or warm-temperate, not unlike that of Virginia or the Carolinas at the present time, and the presence of numerous species of figs, a supposed bread-fruit tree, cinnamons, bays, and other southern plants indicates that it may have been almost subtropical. However, the conditions that were favorable to this seemingly subtropical growth may have been different from the conditions now necessary for the growth of similar vegetation. It may be that these supposed subtropical plants were at that time so constituted as to grow in a temperate land, and that they may have become tropical in recent times. Following this general line of thought it may be said that although the Tertiary vegetation of the Yellowstone National Park would now be regarded as indicating a temperate or even warmer climate, the actual climate may not have been subtropical. It is certain, however, that the conditions were very different from those now prevailing in the park.

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Last Updated: 02-Apr-2007