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.
[Illustration: Fig. 1.—Fossil Logs in Petrified Forest National
Monument, Apache County, Arizona.]
[Illustration: Fig. 2.—Upright fossil trunk in Gallatin Mountains,
Montana.
Courtesy of E. C. Alderson.]
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.
[Illustration: Fig. 3.—Upright trunk and “hoodoo” in Gallatin
Mountains, Montana.
Courtesy of E. C. Alderson.]
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 coming 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 outburst—a 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.