Ancient Rocks
The oldest rocks in the park were formed when crustal plate movements caused deeply buried sea sediments to fall under intense heat and pressure. This resulted in the creation of metamorphic rock (schist and gneiss). These rocks are thought to be 1.7 billion years old. Later the rocks were intruded by magma which cooled about 1.4 billion years ago, forming crystalline igneous rock (mostly granite).
Uplift and Erosion
Approxamately 70 million years ago, the uplifting of rocks in Rocky Mountain National Park began. Giant blocks of ancient crystalline rock, overlain with younger sedimentary rock, were fractured and thrust upward. Even as the uplift occured, small streams eroded sedimentary materials. After the sedimentary rocks were gone, the streams continued to erode the ancient rock until only a few remnants remained to oversee the gently rolling landscape. About 25 million years ago, volcanic deposits of younger igneous rock were laid upon the older Precambrian formations. These volcanic rocks can be seen in the Never Summer Mountains, on Specimen Mountain, and along Trail Ridge Road at Lava Cliffs. Faulting and upwarping continued to lift the Rocky Mountain Front Range as much as 5,000 feet. At 2 million years ago, the present elevations above 12,000 feet had been reached. Sedimentary layers on the eastern plains had tilted towards the mountains. The stream erosion of these rock formations produced hogback ridges near the present towns of Lyons and Loveland. Differential movement along faults disrupted the drainage patterns, creating higher mountains, waterfalls, and large valleys such as in Estes Park and the Kawuneechee Valley.
Glaciation
While the uplifts and volcanic activity provided the major geologic building blocks for the park, it was the glaciers that shaped the landscapes the way they are today. Two million years ago, during the Pleistocene "ice age," streams cut V-shaped valley drainages into hard rock before the climate cooled. The the highest of the valleys, deep snow compacted into ice and then flowed as glaciers, widening the valleys into U-shaped valleys. The converging glaciers flowed into the lower valleys where the ice melted and dropped off debris that was taken from the mountains above. Debris deposits anlong the valley sides are known as lateral moraines. The material left at the farthest reach of the glacier are known as terminal moraines.
During the last major period of glaciation, which started about 28,000 years ago, glaciers from Forest Canyon, Odessa Gorge, and other tributary valleys all flowed together, forming a large glacier which melted in the area now called Moraine Park. This enormous glacier left clear lateral moraines along the south and north sides of Moraine Park and a terminal moraine against Eagle Cliff Mountain to the east. Glaciers similar to this one left moraines in what is now Glacier Basin, Horseshoe Park, and the Kawuneeche Valley. Bear Lake and Grand Lake are both naturally moraine-dammed lakes.
Present Landscape
Today, the Precambrian granites and metamorphic rock predominate throughout the central and eastern sections of the park. A few remnants of sedimentary rock can be found amoung the volcanic formations in the Never Summer Mountains.
Steep-sided, semicircular bowls (known as cirques), often filled with snow, for the tops of the U-shaped valleys. Chasm Lake, below the east face of Longs Peak, rests in cirque. From Trail Ridge Road, one can easily see a cirque on Sundance Mountain. Many more can be seen from Bear Lake Road. Glacial erosion also left scratches (called striations), grooves, and polished surfaces on some of the rocks. A good example of these marks can be found along Old Fall River Road. The few small glaciers and snowfields in the park today only hint at the Ice Age giants that came sweeping through the park.
Some areas of the park on the high mountaintops and at lower altitudes were not glaciated. The Twin Owl and Gem Lake Trail areas have interesting shapes by millions of years of non-glacial erosion.
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