These canyons were filled by glaciers at intervals during the million
years of the ice age. This period saw the formation of vast ice fields
over much of northern North America. The causes of the ice age are
complex, but its effects on our landscape are marked and convincing.
[Illustration: Sandstone hogbacks flank the mountains on the east.
Scene near mouth of Big Thompson Canyon, west of Loveland.]
[Illustration: Remnants of an erosion surface on peaks south of
Trail Ridge Road.]
[Illustration: High mountain lakes are set among the scars of
glacier excavation. Arrowhead Lake, in Gorge Lakes Canyon.]
[Illustration: Great forests, high peaks, and cool summers add to
the appeal of Rocky Mountain National Park.]
Every large high-altitude canyon in what is now Rocky Mountain National
Park became filled with snow, much of which, under pressure, turned to
ice. The glaciers thus formed, moving under their own great
weight--slowly, but with tremendous power--broadened, deepened, and
straightened the twists and turns of the original river-cut valleys,
and, bit by bit, scooped out bowls, or cirques, at the glacier sources.
These glaciers quarried and removed untold millions of tons of rocks
from the upper reaches. Many of the cliffs and lakes of the park are the
results of excavating done by the mountain glaciers.
These glaciers were entirely local; they did not extend down to the
plains in this region. At what is now an altitude of about 8,200
feet--just below present-day Moraine Park, Horseshoe Park, Wild Basin,
and a few miles below Grand Lake in the Colorado River valley--the
glacier fronts melted as fast as the ice advanced. It was there that
most of the rock debris plucked from higher up was dropped. Piles of
rock are scattered over most of the meadowlands of this general
altitude. These rock deposits are called moraines--ridges and heaps, or
scattered masses, of unsorted rock debris dumped where they settled from
the melting ice. Classic examples of moraines may be seen in Moraine
Park, named for these special features. You can learn more about them at
the Moraine Park Visitor Center.
When climatic changes caused the glaciers to melt back faster than they
advanced, the moraines, like modern dams, formed lakes behind them
whenever stream drainage from the shrinking glaciers was checked.
Several such lakes, now silted in and changed to green meadows, occupied
lower regions in the park. There are good examples of these lakes in
Horseshoe Park and Moraine Park.
[Illustration: Bierstadt Ridge, a huge glacial moraine, seen en
route to Bear Lake.]
The glaciers invaded the park valleys several times during the ice age.
Usually, two distinguishable ages of moraines can be seen; the older and
more extensive one is made up of well-weathered, "rotten" boulders and
finer material, while the newer ones are scarcely altered at all. It is
thought that the last great glaciers retreated only some 12,000 years
ago. Indians lived on the plains at that time!
[Illustration: Andrews Glacier, late in season. Note crevasse in
upper middle of glacier.]
It is by no means certain that the glacier age is entirely a thing of
the past. Five small glaciers, or "glacierets," of geologically recent
origin--Taylor, Andrews, Tyndall, Rowe, and Sprague--exist today in the
park. They are much smaller than the earlier glaciers; but they are ice
masses, and they are moving (though very slowly); so they are glaciers
by definition. They are accessible only by arduous foot travel, but the
first three are visible from heavily traveled roads and trails of the
park.
The story of the geological events, as we have seen, is long. The
landscape of today, we now realize, is transient. It is the contemporary
product of processes that have been working day and night throughout
geologic time. These processes will certainly go on and on, and
inevitably will continue to change the landscape. Each year sees some
little modification here and there. These small changes are not linked
in our thinking with the vast sweep of geologic time, probably because
our own lives are so very short. With some reflection we seem to catch
glimpses of eternity as we examine the ancient gray cliffs and mighty
peaks of Rocky Mountain National Park.