THE FOSSIL FORESTS OF THE YELLOWSTONE NATIONAL PARK.
By F. H. KNOWLTON,
United States Geological Survey.
Isolated pieces of fossil wood are of comparatively common and widespread occurrence, especially in the more recent geological deposits of the West. Not infrequently scattered logs, stumps, and roots of petrified or lignitized trees are brought to light, but only exceptionally are they so massed and aggregated as to be worthy of the designation of fossil forests. Examples of such are the celebrated fossil forests of relatively late geological age near Cairo, Egypt, the huge prostrate trunks in the Napa Valley near Calistoga, Cal., and the geologically much older and far more extensive forests now widely known as the Petrified Forest National Monument in Apache County, Ariz. But in many respects the most remarkable fossil forests known are those now to be described in the Yellowstone National Park. In the forests first mentioned the trunks and logs were all prostrated before fossilization, and it is perhaps not quite correct to designate such aggregations as veritable fossil forests, though they usually are so called. In the fossil forests of Arizona, for example, which are scattered over many square miles of what is now almost desert, all the trunks show evidence of having been transported from a distance before they were turned to stone. Most of them are not even in the position in which they were originally entombed, but have been eroded from slightly higher horizons and have rolled in the greatest profusion to lower levels. As one views these Arizona forests from a little distance, with their hundreds, even thousands, of segments of logs, it is difficult to realize that they are really turned to stone and are now exhumed from the earth. The appearance they present (see fig. 1) is not unlike a "log drive" that has been stranded by the receding waters and left until the bark had disappeared and many logs had fallen into partial decay. Trunks of many sizes and lengths are now mingled and scattered about in the wildest profusion, and the surface of the ground is carpeted with fragments of wood that have been splintered and broken from them. In the Yellowstone National Park, however, most of the trees were entombed in the upright position in which they grew, by the outpouring of various volcanic materials, and as the softer rock surrounding them is gradually worn away they are left standing erect on the steep hillsides, just as they stood when they were living; in fact, it is difficult at a little distance to distinguish some of these fossil trunks from the lichen-covered stumps of kindred living species. Such an aggregation of fossil trunks is therefore well entitled to be called a true fossil forest. It should not be supposed, however, that these trees still retain their limbs and smaller branches, for the mass of volcanic material falling on them stripped them down to bare, upright trunks.
The fossil forests of the Yellowstone National Park cover an extensive area in the northern portion of the park, being especially abundant along the west side of Lamar River for about 20 miles above its junction with the Yellowstone. Here the land rises rather abruptly to a height of approximately 2,000 feet above the valley floor. It is known locally as Specimen Ridge, and forms an approach to Amethyst Mountain. There is also a small fossil forest containing a number of standing trunks near Tower Falls, and near the eastern border of the park along Lamar River in the vicinity of Cache, Calfee, and Miller Creeks, there are many more or less isolated trunks and stumps of fossil trees, but so far as known none of these are equal in interest to the fossil forest on the slopes of Specimen Ridge.
The fossil forests are reached over a road from the Mammoth Hot Springs, or from Camp Roosevelt near Tower Falls, and they are in their way quite as wonderful and worthy of attention as many of the other features for which the Yellowstone National Park is so justly celebrated.
Recently another extensive fossil forest has been found on the divide between the Gallatin and Yellowstone Rivers in the Gallatin Range of mountains in Park and Gallatin Counties, Mont. This forest, which lies just outside the boundary of the Yellowstone National Park, is said to cover 35,000 acres and to contain some wonderfully well preserved upright, trunks, many of them very large, equaling or perhaps even surpassing in size, some of those within the limits of the park. Two of the best preserved of these trunks are shown in figures 2 and 3, which are here reproduced by the kindness of Mr. E. C. Alderson, of Bozeman, Mont.
In the beds of the streams and gulches coining down into the Lamar River from Specimen Ridge and the fossil forests one may observe numerous pieces of fossil wood, which may be traced for a long distance down the Lamar and Yellowstone Rivers. The farther these pieces of wood have been transported downstream, the more they have been worn and rounded, until ultimately they become smooth, rounded "pebbles" of the stream bed. The pieces of wood become more numerous and fresher in appearance upstream toward the bluffs, until at the foot of the cliffs in some places there are hundreds, perhaps thousands of tons that have but recently fallen from the walls above. One traversing the valley of the Lamar River may see at many places numerous upright fossil trunks in the faces of nearly vertical walls. These trunks are not all at a particular level but occur at irregular heights; in fact a section cut down through these 2,000 feet of beds would disclose a succession of fossil forests (see fig. 4). That is to say, after the first forest grew and was entombed, there was a time without volcanic outbursta period long enough to permit a second forest to grow above the first. This in turn was covered by volcanic material and preserved, to be followed again by a period of quiet, and these more or less regular alternations of volcanism and forest growth continued throughout the time the beds were in process of formation.
While these fossil forests were growing and being entombed, much of the area now within the limits of the park, as well as large adjacent areas, was the scene of tremendous geologic activities. After the Cretaceous period (see diagram p. 30), there was a time of great volcanic activity, which appears to have lasted until perhaps the beginning of the glacial epoch. There were many active volcanoes just east, north, and west of the park, and some in the park itself. From these volcanoes vast quantities of material were poured out, building up in places whole mountain ranges. Thus the major portion of the great Absaroka Range, just east of the park, as it appears to-day, was built up of volcanic material.
Mr. Arnold Hague gives the following graphic account of this and adjacent areas:
Within the park there is evidence of similar volcanic activity, and it is clear that the basin between the encircling ranges was filled to its present elevation by volcanic flows, which formed the present park plateau. The area within which the fossil forests are now found was apparently in the beginning an irregular but relatively flat basin, on the floor of which after a time there grew the first forest. Then there came from some of the volcanoes, probably those to the north, an outpouring of ashes, mud flows, and other material which entirely buried the forest, but so gradually that the trees were simply submerged by the incoming material, few of them being prostrated. On the raised floor of the basin, after a time, the next forest came into existence, only to be in turn engulfed as the first had been, and so on through the period represented by the 2,000 feet or more of similar beds. The series of entombed forests affords a means of making at least a rough estimate of the time required for the up-building of what is now Specimen Ridge and its extensions. (See p. 29.)
During the time this 2,000 feet of material was being accumulated, and since then to the present day, there has been relatively little warping of the earth's crust at this point; that is, the beds were then, and still are, practically horizontal, so that the fossil forests, as they are being gradually uncovered, still stand upright.
When the volcanic activities had finally ceased, the ever-working disintegrating forces of nature began to tear and wear down this accumulated material, eroding the beds on a grand scale. Deep canyons and gulches have been trenched, and vast quantities of the softer materials have been carried away by the streams and again deposited on lower levels or transported to great and unknown distances.
As the material in which the fossil forests are now entombed consist of ashes, mud flows, breccia, and the like not all the beds are of the same texture and hardness, so that erosion has acted unevenly on them and has produced many peculiar rock forms. The grotesque so-called "hoodoos" have been carved out in this manner. The fossil trunks, being usually harder than the surrounding matrix in which they are embedded, have more firmly resisted erosion and now project to different heights above the general level. In exposed beds that are nearly or quite horizontal, disintegration has acted at nearly equal pace on the trunks and on the matrix, so that the trunks are nearly or quite on a level with the surrounding surface. On steep hillsides, however, from which all loose material is easily and quickly removed, some of the fossil trunks stand up to a height of 20 or 30 feet. If the beds had been tilted at a considerable angle, these trunks could project from the surface for only a short distance before their weight would break them off, showing again the remarkably stable conditions that have continued since the trunks were covered up.
Last Updated: 02-Apr-2007