A Waterton-Glacier Story: The Work of Ice

Many people who visit Waterton Glacier International Peace Park (WGIPP) for the first time expect to see large glaciers with snouts that come right up to the edge of the road. Instead, they catch long-distance glimpses of small glaciers high in the mountains. Visitors who hike have an opportunity to examine the remnants of glaciers that were much larger in times past.

Since the Ice Age began approximately two million years ago, at least four major continental ice sheets have advanced into this area and then receded. As the continental glaciers approached from the north and east, glaciers began to grow and advance in the mountains. The ice got so deep that it nearly covered the tops of the mountains and on several occasions the resulting valley glaciers joined with continental ice sheets on the east side of what is now W-GIPP.

Glacier National Park, the U. S. portion of WGIPP, was named for the glacially-carved features that give character to the mountain landscape. Today fewer than fifty small glaciers still exist in the park. By studying and comparing the small remaining park glaciers with large glaciers that are still dynamic agents in other parts of the world scientists are able to understand what occurred in this area so many ages ago.

What is the work of ice? What is a glacier? The term “glacier” is derived from the French word “glace”, meaning ice. Some two million years ago the climate in this area began to grow cooler. More snow accumulated in the mountain valleys than melted during the warmer months. After a time the accumulated snow began to contribute a further chilling effect to the weather. As the snow got deeper, it compressed. The underlying snow began to change or re-crystallize into a dense form of ice called firn. By the time the firn reached a depth of about 150 feet it was solid ice.

Because the snow accumulation was heaviest at the higher ends of the mountain valleys, most of the growth originated there. Pulled toward a lower elevation by gravity, the newly formed glaciers began to move slowly down the valleys.

As the front of the glaciers moved to lower elevations, snow continued to accumulate at the head of the valley. Soon the small glaciers became giant valley glaciers. Eventually the accumulation of snow and ice became so extensive that at times only the highest peaks in the park remained above the glaciers.

The base of a glacier is under so much pressure that it behaves like soft plastic, oozing around and sliding over the underlying bedrock and soil. Glacial ice fills every crack and moves house-sized boulders with ease. Once a rock or boulder has been enveloped in the base of a glacier, it becomes a tool for carving and abrading the surface over which it moves. The net result is a relatively straight and flat U-shaped valley where an uneven V- shaped, stream-carved valley previously existed.

Not only does a glacier carve the valley floor, it also plucks material from the surrounding valley walls. While the base of the glacier excavates deep into the bedrock, and the flanks of the glacier pluck and gouge the surrounding slopes, the tail of the glacier continues to pluck away at the headwall. Seasonal temperature fluctuations cause the glacier to melt against the headwall leaving a narrow gap between rock and ice in summer. The gap fills with meltwater that turns to ice each winter eroding the rock by expanding in tiny cracks. This bergschrund, or gap area, undercuts the headwall to the point where the top of the headwall actually overhangs its base. Eventually, the overhang collapses onto the glacier and the process begins again.

Many glaciers move as slowly as a few centimeters a day, while a few large Alaskan glaciers can travel as fast as 150 feet in a day. The glacier does not move as a solid unit. Because of resistance at the base and along the valley walls, the flow of ice near the surface and center of a glacier is often faster than at the bottom and sides. The cracks that result when upper layers of the ice move faster than lower layers are called crevasses. They can be hundreds of feet deep and many feet wide.

Eventually, the snout or toe of a glacier reaches a point where lower elevation or warming temperatures create an equilibrium between annual snowfall and snowmelt. The glacier can advance no further. In the event of a climatic warming trend, the annual snow melt may exceed the amount that falls, and a glacier begins to recede. Most of WGIPP’s glaciers have shrunk dramatically in the last century.

A glacier carries a tremendous load of eroded material in a constant conveyor process toward the toe and edges of the glacier. Ice at the toe melts and runs off as glacial outwash. New ice is constantly being replaced near the head of the glacier. Rocks break up much more slowly than ice eventually ending up at the toe or sides where they are deposited as glacial till. Till consists of a jumble of rocks, gravel, dirt or other debris that may have been picked up by the glacier. Piles of till along the margins of a glacier are called moraines.

If the moraine occurs at the point of farthest advance of a glacier, it is called terminal moraine. Sometimes a glacier will retreat up a valley and stabilize temporarily at various stages of the recession. In such a case it may leave a series of what appear to be terminal moraines, but are referred to as recessional moraines. If a glacier retreats steadily, it leaves a variety of till and outwash formations along the path of recession back up the valley. While a glacier is moving, till tends to work its way to the sides of the valley and be deposited along the glacier’s flanks. During the lifetime of a glacier, the amount of till that builds up along its sides can be impressive. The resulting long, fertile hills, called lateral moraines, are conspicuous along major valley edges in the International Peace Park. The older lateral moraines are most often recognized by dense conifer forests that cover them.

All glaciers have melt water running from their snouts during warmer seasons. The melt water carries a load of sediment for deposit along an outwash plain. Depending upon the volume and speed of the outwash stream, sediments are sorted and deposited along the floor of the plain. Respective weights of the various particles determine where they will be deposited-- near the snout of the glacier or further downstream. Outwash streams are frequently forced to change their courses because they fill with these sediments. The net result is a network of braided streambeds on the outwash plain.

The lightest, smallest sedimentary particles may be carried a long way until the stream has slowed considerably. This pulverized rock is appropriately called glacial flour. Glacial flour is particularly evident in the remnant tarns or mountain lakes that lie in the abandoned cirques near the headwalls of glaciers. The flour is actually the grist left from the grinding force of the glacier. Remnant icefields beneath the headwalls in the Peace Park continue to color the lakes with their flour. In the park every lake has a slightly different color depending upon the makeup and mixture of rocks. Some lakes have a white tint that actually suggests flour, but more often they are various shades of blue and green.

A number of curious formations left by retreating mountain glaciers are found on both sides of WGIPP. Many of the lakes among the foothills on the Blackfeet Reservation and in the Flathead Valley are called kettle lakes. They were formed when a melting mass of glacial ice remained in a depression after the main body of the glacier had retreated further into the mountains. When the mass finally melted, a depression remained. In time, it filled with water. Teardrop-shaped drumlins, or hills, were left where the bedrock resisted the gouging of an advancing glacier. Softer, surrounding rock was worn away and this elevated nucleus collected sediments that built up around them as the glacier retreated. Eskers, elongated hills, were formed by sediments deposited by streams flowing in tunnels beneath the ice. Kames, another form of depositional hill, were formed when openings developed in stagnating ice. Glacial erratics, the probable source of the matched legends of Coyote and Napi punishing a rock, have long been a source of fascination. Erratics, often found in open country many miles from any possible source, are either unusually large boulders found among smaller till or large rocks that were ice-rafted and deposited on the floor of glacial outwash lakes.

The most dramatic and obvious glacially carved features are found along the courses of U-shaped troughs left by the now-departed valley glaciers. The high sharp peaks are called horns. Serrated narrow ridges left between the headwalls of two adjacent glaciers are called aretes. Along the sides of the main glacial troughs, hanging valleys are often found where smaller tributary glaciers once abutted the main glacier. Beneath the headwalls of each of these tributary glaciers, one often finds a depression called a cirque which may hold a tarn or cirque lake. Along the course of the main glacial trough there are a series of truncated spurs, cliffs bulldozed into the sides of gradual mountain slopes.

Last updated: February 24, 2015

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