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III. Erosional Processes: Glaciation and Stream Formation

We have been concentrating primarily on mountain building processes, but erosional processes influence mountain shape and form as well. If the rate of erosion equaled the rate of mountain building in Colorado, there would be no mountains. Our state would be as flat as Kansas. Erosion is defined as the removal of mineral (rock) particles from exposed surfaces by the impact of a fluid (water, air, or ice). Erosion is the process of removing weathered material. Weathering may involve a chemical reaction that weakens the bonds that hold the rock together (chemical weathering), or may be due to physical processes that break the rock apart. One very powerful form of physical weathering is called freeze-thaw. When water within cracks in a rock freezes, it expands. This expansion is so forceful that it may break the rock apart. This is one of the processes that causes streets and sidewalks to crack. In the mountains, where there is a lot of freezing and thawing going on, physical weathering is an important process.

Mass-wasting is the term geologist use for the gravitational downward movement of weathered rock debris. When the strength of a slope is overcome by gravitation, the slope fails. Rock chips, boulders and pebbles, no matter where they first fall, eventually reach streams by a slow continual downward pull of gravity. This movement is aided by freeze and thaw processes, and by lubrication from water and snow. It is in this way that narrow, close-walled canyons over immense spans of time widen into valleys. Think of the narrows at the mouth of the Big Thompson Canyon. Do you think the canyon will stay that constricted forever? What factors determine how fast the canyon walls will flake away? (Hint: look at the hogback Figure on the Colorado Front Range homepage. Do the different types of rock on the top of the hogback appear to be weathering at different rates?). Two agents of erosion that both contribute to weathering, erosion, and rearranging of the material created are glaciers and streams.

Glaciers as Erosive Agents

What is a glacier? Glaciers are bodies of ice and snow that persist at a site year round. When the amount of snow falling on an area exceeds the amount of snow melting in that area, a glacier results. The rate of melting is determined by local climate, while the rate of snow falling depends on both local and regional climate patterns. When the atmosphere is warmer over a region, more melting occurs and glaciers cover a smaller proportion of the landscape, or do not occur at all. In Colorado, glaciers used to cover much more of the landscape than they do today. Today we can still find glaciers throughout the high country, but they are limited to cool places at very high altitudes (>9000 ft). What do glaciers have to do with mountain formation you may ask? They actually play a huge role in what we see when we drive or hike through the high-country. Glaciers tear mountains down. They are one of the many erosive agents that scour, grind, scrape, and plow mountain landscapes into the rugged form we see today. Sculptors so to speak.

Photograph of a glacially carved mountain landscape near Crested Butte, Colorado.

Is ice hard enough to carve through granite? No! Well then, how do glaciers sculpt and tear down mountains? Ice is very heavy. Under the force of gravity, glaciers begin to move downhill. The ice actually deforms under the high pressure and wraps around rocks and gravel (even boulders). The rocks and debris is carried downhill along with the glacial ice. These materials are hard enough to scrape and grind the bedrock. The tremendous weight of rock and ice in the glacier grinding together make deep gouges in the rocky slopes and carves many features unique to glaciation. Cirques, spurs, tarns and hanging valleys are all evidence of past glacial activity. Over long periods of time, glaciers can carve valleys, create lakes, and literally "move mountains"! When they "bull-dose" through a valley, they push tons of material to the sides of the valley, and at the base of the glacier, they leave big heaps of rock as well. These features are called moraines (lateral and terminal moraines). Glaciers also deposit all the materials that are trapped in the ice when they melt. These material drops to the ground in unorganized heaps. glaciers melt because of warming climate or lack of snow.

The next time you are in the mountains, look at the shapes and forms of the peaks and alpine ridges for evidence of a glacially influenced landscape. Today, most glaciers are retreating and retracing their path back up the mountain. Moraines mark the spot where the glacier stopped advancing and began the retreat. When you drive in the Cache La Poudre Canyon, you can have lunch at a spot called terminal moraine. This was the furthest down the canyon that the glaciers reached during the last ice age (~10,000 years ago).
Now think about mountains in a different way. Instead of looking at the glacially sharpened peaks as the basic unit of a mountain range, think of the valley underneath it as the basic unit of landscape. Check out the bowl below the visitors center on Trail Ridge Road in Rocky Mountain National Park (but wait until the summertime). Streams now flow in many of the valleys that were initially formed by glaciers.

Streams as Erosive Agents

--UNDER CONSTRUCTION--

Photograph of a mountain stream in the Lost Creek Wilderness along Colorado's Front Range.

Look at the alluvial fans at the mouths of the canyons along Colorado's Front Range in the relief map of Colorado.

What happens to all this sediment when the load becomes to or when the stream velocity slows down as the stream exits the mountains. The heaviest particles, usually sand, fall out first . As the velocity decreases more then the silts and clays settle out. This is often why a ridge of sandy material is nearest the river bank and the broad flats of the flood plain have the heavier silty and clayey soils.