The Mountains Come Down – Breaking Down the Rocks  

Key Topics

Introduction

As we look upwards, it's hard to imagine the mountains almost twice their present size – however in their infancy they were almost that large. Slowly, through the processes of weathering and erosion, the mountains have been reduced to a fraction of their original size. The material removed from these mountains has been used to help fill in the many mountain lakes in the area. This will all lead to a very different terrain in a few dozen million years from now.

Beginning upslope, a number of processes have conspired to slowly wear the mountains down. Frost heaving is one of the most critical erosional process. During the day, water flows into cracks and crevices in the rocks and, as the temperature drops with the onset of evening, freezes. Since water expands when it freezes, it forms a powerful wedge which can quarry large pieces of rock. Most of the loose rock or scree slopes so common in the mountains have been formed in this way.

Other processes include the slow carving by glaciers and rivers, both of which tend to wear the rocks down to smaller and smaller particles until they reach the consistency of flour. This rock flour, as well as coarser material, flows downstream (or down-glacier since glaciers are little more than a river of ice) and further helps to carve the valley bottoms. It does this by acting like a giant abrasive, wearing down the soft rocks of the Rockies. This adds more and more material to the river.

One might ask where all of this material ends up? It finds its way into the lakes and oceans downstream. Water moving at a high speed, such as in a steep river, can carry huge amounts of material – Lake Sunwapta at the base of the Athabasca Glacier receives 570 tonnes of material every day during summer runoff. When these fast flowing streams enter any large body of water, their speed drops rapidly and restricts their ability to carry sediment. As a result, the material is unceremoniously dumped at the entrance to the lake, subsequently resulting in the formation of deltas. Lake Sunwapta will likely disappear within the next 10 years. Peyto Lake, on the Icefields Parkway, has a huge delta growing outward from its source. Almost 1/3 of the lake has been filled.

Slowly, the forces of nature are wearing the mountains down from the summits, and filling in the lakes from their stream entrances outwards. This implies a natural preference for flat terrain. If everything stayed consistent for all time, the world would be a rather flat and uninteresting place. However, thanks to constantly changing climates and large scale geological movements, like mountain building, we are treated to unending diversity. As nature slowly sculpts the landscape and works to flatten it, occasionally rapid (geologically), large scale disruptions, like the uplifting of mountains, force it to begin anew.

Landslides in the Rockies

Landslides are one of natures most powerful events. Suddenly, without warning, huge amounts of material may break off a formerly quiet hillside and come thundering down the valley wall. Moments later, the peace is restored, but beneath the dust is a scene of power and devastation. In 1903, in the Crowsnest Pass, the sleeping town of Frank was buried beneath hundreds of tonnes of debris when part of Turtle Mountain released the Frank Slide. The magnitude of this event was sufficient to inspire an impressive interpretive centre designed to showcase this historic event.

Despite their potential devastation, large landslides are as rare as they are spectacular. Most of our mountain slopes are made up of rocks that were originally deposited horizontally, and later thrust up at steep angles to the horizontal. In some cases, the boundary between two layers may provide an area of weakness that may catastrophically release without warning. To complicate matters worse, glaciers cut deeply into the sides of most of our valleys, leaving many of these slabs unsupported. This has occurred along the Icefields Parkway just north of Tangle Falls. The fracture can be clearly seen in the thick layers of pink quartzite which now litter both sides of the highway.

Moraine Lake lies behind a dam of debris caused from a rockslide that came off the Tower of Babel. Some of this debris may have fallen on glacial ice and been moved a short distance before being dumped at its present location.

Numerous other mountain lakes have been similarly formed. Medicine Lake in Jasper National is caused by a large slide from the Queen Elizabeth Range, and Maligne Lake from a slide off the Opal Hills and surrounding slopes.

Agents of Change - How are the Mountains Slowly Shrinking

Chemical Weathering

Many of the rocks that make up the mountains, in particular limestone, are susceptible to chemical reactions with materials carried in the air and water. In some cases, a chemical reaction to the water changes the material. In others, the water may merely dissolve the substance. Most of our caves are formed by limestone dissolved by the action of water. Finally, the oxygen in the air around us may oxidize minerals in the rock. The reddish colour of the rocks making up the exterior walls of the Banff Springs Hotel is the result of iron in the shales slowly oxidizing once exposed to air.

Freeze and Thaw

Water is unique. As it freezes, it expands in volume. In the mountains, snow melts during the day, and the resulting water flows into cracks in rocks. Overnight, as temperatures drop, the water expands and freezes. This expansion provides an incredible wedging force and slowly chips away at the rock face. Most of the local scree or loose rock slopes were formed by the slow action of ice wedging. As water freezes, it expands 9%, providing the sheering force of dropping a shot put 3 m. onto the rock.

Glaciation

Glaciers take the landscape and sculpt it. Sharp v-shaped valleys are smoothed into rounded u-shaped valleys. Glaciers near the summits create sharp horn peaks like Mount Assiniboine. In other cases glaciers on two sides of a ridge can create a sharp knife-blade ridge like Nihahi Ridge in the eastern part of Kananaskis Country.

Avalanches

As snow accumulates high in the mountains, eventually the excess is released in the form of avalanches. Since these releases will follow the most efficient route, they normally follow the same path year after year. Some years may widen the path if snowfalls are large, other years may allow tree growth to slowly try to reclaim the path. The signs are always evident, and these paths provide opportunities for lush summer growth.

Running Water

Water is the king of erosion. Nothing carves up a landscape like the action of water. Even glaciers are simply another form of this versatile material. As water begins to move, it picks up material in the form of gravel and debris. As these sediments are carried within the water, they provide the abrasive needed to begin carving river channels. As the water continues to move, it picks up increasing amounts of water and sediment, and the process continues.

Soil Creep - click to see a larger imageSoil Creep

Very subtle, creep can be almost undetectable. It is normally found on weathered mountain slopes. Under the force of gravity, the material slowly begins to slide downhill, and eventually the signs of creep become evident. Tree trunks curve upward as the creep had caused them to tilt downhill, and they in turn corrected for this. Fence posts and hydro poles may also begin to lean downslope as the effects of creep increase.

Slumping

In many situations, a steep hillside will show a spoon-shaped depression within which the material has begun to slide downhill. This slump, as it is known, occurs along a distinct fracture zone, often within materials like clay, that once released, may move quite rapidly downhill. In some cases the slump is caused by water beneath the slope weakening it. In many cases it is simply the result of poor engineering along highways where it is a regular occurrence.

Rockslides

Sometimes, as layers of rock are steeply uplifted, the bonding of one layer to another may be weakened by the action of water or other agents of erosion. As the force of gravity constantly pulls down on the surface of a steep slope, there may eventually be a failure. As a fracture occurs at the top, a layer may slide down suddenly creating a large rockslide.

Wind

In the Rockies, wind tends to play a minor erosional role. In area’s where the wind constantly blasts, as in the Bow Valley, the wind may add to the effects of other forces.

Biological Action

The orange and green lichens that coat many of our rocks are slowly breaking them down. With the ability to slowly break down their limestone home, they begin the lengthy process of soil creation. On a larger scale, the trees that seem able to grab a tenuous roothold on the steepest of slopes , do so by jamming their roots down any available crack. This creates a classic wedging effect which may break off large pieces of rock

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All Material © Ward Cameron 2005