The geological story of Rocky Mountain National Park is a long one. Most
of its details are lost in the passage of hundreds of millions of years.
Some of the story has been put together by scientists from bits of
evidence scattered here and there. The evidence strongly indicates a
certain chain of events, but no eyewitnesses are available to confirm
the deductions. Few of these events can be proved to everyone's
satisfaction; we can but pass on to you some determinations that
geologists have made.
Most of the rocks which you see in the park are crystalline and very
ancient. The gneiss and schist were, in part, once sediments formed in
seas, perhaps a billion years ago, under conditions about which there is
little knowledge or general agreement. These sediments were buried
beneath thousands of feet of other sediments, cemented and hardened into
layers of sedimentary rock and later squeezed, crushed, and elevated by
slow, ceaselessly working earth forces that produced mountains even in
that ancient time. During this period the sedimentary rocks were changed
to harder metamorphic rocks, probably because of deep burial under
tremendous pressure and considerable heat. Masses of molten rock welled
up into these earlier deposits and hardened under the earth's surface.
This later (though still very ancient) intrusive material is now exposed
granite in many parts of Rocky Mountain National Park.
[Illustration: Rocks once buried miles deep are now exposed on Longs
Peak, at an altitude of more than 14,000 feet.]
These ancient mountains were gradually worn away by wind, rain, and
other agents of erosion, which must have attacked the surface of the
earth as vigorously then as now. With the passage of millions of years,
these mountains were reduced to a lowland. Another sea gradually lapped
over the land where mountains had been, and once again sediments were
dropped in its bottom. This new invasion of the ocean affected the park
region during many millions of years in which the dinosaurs dominated
the earth.
In response to little-understood rhythms of the earth's crust, which
have lifted mountains ever so slowly at great intervals all over the
world, the seas drained away as the crust rose again, and the rising
land once more became subject to the ceaseless attack of erosion. This
uplift--which began about 60 million years ago--originated the system of
mountain ranges and basins that today give Colorado its spectacular
scenery and much of its climate. This great period of mountain-making is
called the Laramide Revolution, from its early recognition in the
Laramie Basin region of Wyoming.
The Front Range, of which this park preserves a choice sample, was
buckled in the fashion of a great long wrinkle in a carpet. This "roll"
of rock was about 200 miles long and some 40 miles across. In its
earlier stages it was covered by the arched-up sediments, but, as time
passed and erosion continued, the inner core of earlier crystalline
rocks was exposed once again. Today, all traces of the former thick
mantle of sedimentary beds are gone from the park. They are still
present beneath the plains to the east and the basins to the west, and
the cut-off ends of some of them now lie exposed in a tilted position
against both east and west flanks of the mountains. The sandstones of
some of the hogback ridges crossed by the approach roads from Lyons and
Loveland are a part of this once continuous overburden.
Uplift continued intermittently during many millions of years. In the
western section of the park, volcanic eruptions took place. Specimen
Mountain is the remnant of a volcano; some of its flows are seen today
as the cliffs behind Iceberg Lake, on Trail Ridge Road. Great sheets of
lava and other volcanic rocks piled in layers now make up much of the
Never Summer Range. Eventually, these rocks, too, will be stripped away
by the relentless work of erosion; this will require millions of years.
An unusual feature of the landscape here is the rolling, sometimes
flattened character of many mountain summits. Trail Ridge Road crosses
several miles of one of these summits--a gently rolling upland above
11,000 feet. These mountaintops appear to be all that is left of an old
land surface that once may have been continuous far eastward over the
area occupied today by the Great Plains. Such surfaces, of which the
mountains in the park show many good remnants, are called peneplains.
Their presence atop the mountains is a part of the evidence suggesting
that the range had been worn down by erosion to a fairly flat upland a
few million years ago. Then renewed uplifting occurred, and streams
draining the highland gradually cut canyons two or three thousand feet
into the elevated surface.