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. 28), 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 west, 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.
[Illustration: Fig. 4.—Ideal section through 2,000 feet of beds of
Specimen Ridge, showing succession of buried forest. After Holmes.]
Mr. Arnold Hague gives the following graphic account of this and
adjacent areas:
From one end to the other the Absarokas present a high, imposing
plateau, with elevations ranging from 10,000 to over 12,000 feet above
sea level. The entire mass is made up almost exclusively of Tertiary
igneous rocks. * * * Degradation of the mass has taken place on a
grand scale. Vast quantities of volcanic ejectmenta have been removed
from the summit, but no reliable data exist by which the amount can be
estimated even approximately. All the higher portions have been
sculptured by glacial ice. Enormous amphitheaters have been carved out
of the loose agglomerates, and peaks, pinnacles, and relics of great
table-lands testify in some measure to the forces of erosion. The
plateau is scored by a complete network of deep valleys and gorges,
which dissect it in every direction, and lay bare the structure of the
vast volcanic pile.[1]
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 upbuilding of what is now Specimen Ridge and
its extensions. (See p. 27.)
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 end 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.