Towering mountain peaks, snow-filled valleys, open meadows, and jagged rock sculptures fill the eye at Rocky Mountain National Park. Although the standard terrain-creating forces of uplift and water erosion that are responsible for the scenic beauty of much of the Western national parks were key players here, another force that was not a factor at Zion, Bryce Canyon, and the Grand Canyon was also at work -- glaciers.
Rock formations in the park are among the oldest in the United States, having been formed at least 1.7 billion years ago, when movements of the earth's crust created heat and pressure on deeply buried sediments that had been left from an earlier inland sea. This produced metamorphic rock, and then about 1.4 billion years ago volcanic activity deep underground created granite and other igneous rock. As the climate changed, shallow inland seas came and went, leaving deposits that over vast amounts of time became layers of sedimentary rock.
About 70 million years ago, a period of uplifting began in which big chunks of igneous rock, along with the younger sedimentary rock on top of it, fractured and were forced upward. Rivers carved away much of the sedimentary rock and began eroding the granite. Then, some 25 million years ago, volcanoes deposited more igneous rock on top of the older rock.
The earth's crust continued to shift and crack, lifting these layers of rock so that by about 2 million years ago, the peaks that form today's highest mountains were in place. Faulting and stream erosion continued, as earth movements fine-tuned the shape of the mountains, and streams dug V-shaped valleys.
Then Mother Nature got serious. The Pleistocene Epoch (the Ice Age) arrived, temperatures dropped, and deep snow in the higher-elevation valleys was compacted into ice, creating glaciers that covered more than three-quarters of what is now the park. As the glaciers moved downhill under the force of gravity, they ripped into the sides of the V-shaped river valleys, transforming them into U-shaped valleys and carrying off rocks, dirt, and anything else that got in their way. Then, when the glaciers reached lower and warmer elevations, the ice began to melt, dropping the debris it had picked up on the way downhill. Geologists call deposits left along the sides of the valleys "lateral moraines," and deposits at the front of the glacier -- its farthest point downhill -- are called "terminal moraines."
At least four periods of glaciation occurred, interspersed with warmer, drier weather. The latest, a mere 28,000 years ago, is credited with creating the landscape in the Moraine Park section of the national park, where we can clearly see lateral moraines to the north and south, and a terminal moraine to the east. The remains of glacial deposits can also be seen in Glacier Basin, Horseshoe Park, and the Kawuneeche Valley; and Bear Lake was created when a terminal moraine plugged up a valley.
When exploring the park today, we see several billion years of geologic history. The oldest rocks -- metamorphic and the earliest igneous rocks -- form the higher elevations of Flattop Mountain and Trail Ridge. Younger igneous rock, some 25 million years old, can be seen at Lava Cliffs along Trail Ridge Road, on Specimen Mountain, and in the Never Summer Mountains. Both the Kawuneeche Valley and the valley where Estes Park is located were created by movements of the earth's crust some 2 million years ago.
Semicircular bowls, called "cirques," were formed at the tops of glacier-dug valleys. These are often filled with snow. There is a cirque on the side of Sundance Mountain, visible from Trail Ridge Road; and below the east face of Longs Peak, Chasm Lake is in a cirque. Grooves and scratches from glaciers, as well as glacial polishing, are apparent on rock surfaces along Old Fall River Road.
There are still a few small glaciers left in the park, although to most of us they're difficult to distinguish from patches of snow. You'll be able to see, at a distance, several glaciers from the Moraine Park Interpretative Trail, including Andrews, Taylor, Sprague, and Tyndall glaciers. Tyndall Glacier can also be seen from the Sprague Lake Nature Trail, and Andrews Glacier is visible from The Loch and Timberline Falls Trails. During wet years, Andrews Glacier shows evidence of movement down-valley, but it shrinks back in dry years.
A few areas of the park are the result of nonglacial erosion, and those can be seen along the Gem Lake Trail and in the Twin Owls area.
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