Geology of the Park.
Nature seems, from the first, to have designed this region for a
mountain park. In geological chronology it was near the close of the
Cretaceous Period, that the lifting of the great mountain systems of
the West into their present positions was practically finished. In the
formation of these mountains, the general outline of the Yellowstone
Park was already marked out, probably in much more striking features
than at present. A vast rim of mountains, visible now in the Absaroka,
Snowy, Gallatin, Teton, and Snake River Ranges, hemmed in the
extensive area which has since become so famous. Subsequent events
have greatly modified its original form, but the grand outlines at
first determined are still distinctly visible.
In the Tertiary Period, which was next in order of time after the
Cretaceous, changes of the greatest importance occurred, consisting
principally in the outpouring of enormous masses of volcanic material.
The origin of these lava flows has been traced to a few craters, one
of which was near Mt. Washburn, another in the Red Mountain Range, and
a third near the sources of the Lamar River. Mt. Washburn has long
been recognized as part of the rim of an ancient volcano. Both it and
Mt. Sheridan, the two mountains which bore the principal part in
working out the present features of that country, still remain the
most prominent peaks from which the modern visitor can contemplate
the work they have performed.
The outpourings at first consisted of andesitic lavas. They largely
changed the appearance of the mountain ranges and to some extent
filled up the interior basin. The flows were not continuous but were
separated by long intervals of quiet, during which vegetation and the
agencies of erosion were actively at work.
After the cessation of the andesitic eruptions, a quiescent period of
great length ensued. Then came the period of rhyolitic flows, the
centers of volcanic activity being as before Mts. Washburn and
Sheridan. These flows built up the present Park plateau, and
constitute the great bulk of the rocks which the tourist now sees.
Following the period of rhyolitic eruptions, orographic agencies were
active in producing extensive faults or displacements, which in
certain localities radically changed the relative positions of the
rocks.
The last exhibitions of volcanic energy were in the form of basaltic
eruptions. These took place in part through ordinary volcanic craters,
and in part through cracks or seams in the rocks, where they may still
be seen forming extensive dykes. The basalt is of relatively limited
extent, but its striking and picturesque forms wherever it appears
make it more interesting to the tourist than any of the other rocks.
The great variety of superficial appearances which these volcanic
rocks have assumed makes the Park one of the best laboratories in the
world for their study.
The continuance of these various outpourings doubtless extended into
Quaternary time. Then came the Glacial Epoch, the epoch of
wide-spread ice-carving, which still further modified the surface of
the country. The paths of the ancient glaciers have in several
instances been made out and their transported material may readily be
distinguished. One glacier flowed from the Gallatin Range eastward
across Terrace Mountain, where it joined another moving westwardly
from the Absaroka Range. The united streams continued down the
Gardiner and Yellowstone Valleys, in which vast masses of drift still
mark their ancient route.
Glacial action and the common agents of denudation have given the Park
country its present general aspect. These later modifications have
indeed been extensive, and the great variety of form now seen in the
valleys, cañons and hills is the result of their combined action. The
Yellowstone Cañon is a marked example of erosion on a large scale. A
direct result of its formation was the partial draining of Yellowstone
Lake, which had previously existed at a much higher level than now,
and spread over the entire area of the present Hayden Valley.
Since the cessation of the basaltic lava flows there seem to have been
no further lava outpourings in this region. The old volcanoes have
been long extinct and their craters have been modified almost beyond
recognition. But evidences of the power which once worked beneath them
are still abundant, although no longer on so imposing a scale. It is
the hot springs and geysers still in existence which partly render
this region so widely celebrated. That this thermal action originates
mainly in the same source of energy which once poured out the vast
fields of lava, there is no reason to doubt. Many plausible
explanations are advanced to account for the existence of subterranean
heat, but whatever may be its real origin it is doubtless the same for
both classes of phenomena.
The action which is now observable has continued in an ever-decreasing
degree since the close of the lava period. Over vast tracts of the
Park plateau, the rocks are entirely decomposed to unknown depths by
the ascending superheated vapors. Some idea of the extent of this
action may be obtained at the Grand Cañon, which has cut its way a
thousand feet downward into the decomposed volcanic rock without yet
reaching its bottom. The infinite variety of chemic products resulting
from this decomposition has given the Cañon its wonderful coloration.
The same condition largely prevails over the Park plateau. Where now
are dense forests and no superficial evidence of unusual conditions,
there will frequently be found, by digging beneath the surface, the
familiar proof that thermal activity once prevailed there. In
constructing the tourist route from the Upper Geyser Basin to the
Yellowstone Lake, where for nearly the whole distance there is a
complete absence of hot springs, the evidences of former volcanic
activity were found to be abundant.
Facts like these clearly demonstrate that, from a geologic standpoint,
thermal activity in the Park is gradually becoming extinct; and many
persons, taking alarm at this evidence, imagine that the unique
phenomena of the Yellowstone are of an evanescent character, and that
the time is not far remote when they will be known only as matters of
history. There is, however, no occasion for such misgiving. The
present condition is the result of processes that run back probably
for millions of years; certainly for periods of time compared with
which recorded history is insignificant. The same rate of progress
would produce no perceptible change in the lifetime of an individual.
Some who have visited the geyser regions more than once assert that,
after an interval of several years, they observe a marked diminution
in thermal activity. But this is probably because a second visit
ordinarily makes a less vivid impression than a first. The weight of
reliable evidence is certainly the other way. Mr. David E. Folsom,
leader of the Expedition of 1869, made a tour of the Park during the
present season of 1895. He says: "I had a very vivid recollection of
all I saw twenty-six years ago, and I note no important change."
Professor Arnold Hague, probably the best living authority upon the
scientific features of the Park, has compared the hot springs and
geysers by means of authentic records covering intervals of several
years, and he declares that he finds "no diminution in the intensity
of action or in the amount of discharge from the springs and geysers,
since they have been subject to careful observation." While it is
certain that springs are constantly becoming inactive, it is no less
certain that others replace them, and it may be confidently assumed
that the progress toward ultimate extinction will be inappreciable in
our time or for many generations to come.
The distribution of thermal springs over the surface of the earth is
probably more general than is commonly supposed. Only one extensive
area is practically without them, and that is the Continent of
Australia. Africa, also, has very few. But in other parts of the
globe they are found almost without number, ranging from the Equator
to the Arctic Circle, and from sea-level to the lofty table lands of
Thibet.
The three localities, however, in which they abound in such numbers
and magnitude as to attract marked attention are, in the order of
their discovery, Iceland, New Zealand, and the Yellowstone National
Park. In extent, variety, and magnitude of accompanying phenomena, and
in geologic age, the above order is reversed. Iceland has probably the
most famous geyser in the world, principally because it was for a long
time the only known geyser, and consequently received a great deal of
scientific attention; but judging from published descriptions it is
clearly inferior to several now known in the Firehole Geyser Basin.
Three notable features of similarity in these geyser regions are the
presence of volcanic rocks of remote or recent origin; proximity to
the earth's surface of active sources of subterranean heat; and the
presence of a great number of lakes. In all three cases, lava, heat
and water are the characteristic geologic and physical accompaniments
of those particular phenomena which will now be described more in
detail.