Glacial landforms

Glacial landforms are common in many parts of Canada, the United States, Europe, Asia, and many mountain ranges of the world. The material composing these landforms, as well as the shapes of some of the landforms, are essential to many human activities.

Glacial Movement

Glacial landforms are distinguished primarily by shape and composition, which relate directly to the mode of origin of the glacial features. Some landforms originate by erosion or deposition directly by the ice, while others are created by erosion or meltwaters. It is important to distinguish whether ice or its meltwaters produced the landform. Two types of glaciers are common: continental glaciers and valley glaciers. Although some landform features are similar to both, other characteristics are relatively unique.

Glacial ice movement helps explain how these landforms develop. Ice can move because of pressures from great thicknesses of continental ice, or due to gravity down a valley slope. In the case of the latter, the ice always moves forward. Even when a glacier melts backward and appears to be shortening or being made smaller, internally the ice is still moving forward.

Glaciers can erode the material over which they move. If they scrape down to solid bedrock, several glacial features can result. If the glacier is carrying small, hard particles along its base, these particles may scratch the bedrock, creating many roughly parallel striations. If large rocks are being transported at the base of the ice, glaciers can gouge out varisized grooves ranging from a few centimeters to several meters deep. Conversely, if the ice has ground its load into flour-size particles, called rock flour, the flour can polish the bedrock. The orientation of striations and grooves can be used to determine ice-flow direction.

Erosional Landforms

Of all the glacial landforms, perhaps the most spectacular are the erosional products of mountain glaciers. Mountains that have experienced glaciation exhibit some of the world's most breathtaking scenery. Glaciers tend to sharpen peaks and ridges and steepen valley walls, producing avenues for numerous spectacular waterfalls. As snow accumulates near the tops of mountains, the forming ice tends to carve out a basin-shaped depression on the side of the mountain. This basin is called a cirque and is the home base of the glacier. In this basin, more snow will accumulate, thickening the ice, until the ice begins to move out of the cirque and down the valley. If three or more glaciers form around the same mountain peak, the crowding of their cirques around the peak will erode it into a sharper-than-usual pyramidal shape. This type of mountain peak is called a horn. Many of the peaks in the Alps have the word “horn” incorporated into their names; the Swiss Matterhorn is probably the most famous glacial horn. If two glaciers are moving down a mountain in adjacent valleys, the ridge between them may be eroded into a narrower, sharper ridge called an arête.

Mountain glaciers tend to widen the base and steepen the sides of the valleys through which they move, creating U-shaped valleys. Large glaciers have more power to erode more deeply. Small tributary glaciers produce valleys that are not carved as deeply. Therefore, when the glaciers retreat from such an area, the floors of the tributary valleys are left high above the floor of the main valley. Such valleys are appropriately called hanging valleys. If streams are present, they become waterfalls that cascade over the edges of the hanging valleys. U-shaped valleys usually contain strings of lakes that are impounded by irregularities of the valley floor. These lakes are quite picturesque in that the rock flour they contain from glacial meltwaters gives the water a distinctive turquoise color.

Perhaps the most spectacular glacial valleys are fjords, most common in Norway, British Columbia, Alaska, and Chile. A fjord is a coastal glacial valley that was carved out when sea level was lower during times of glaciations. Upon retreat of the ice and subsequent rise in sea level, seawater completely inundated the valley. The water in many fjords is often more than 1,000 meters deep. In some areas where fjords extend inland for considerable distances and intersect with other fjords, it may be difficult to distinguish a fjord from a large lake in a U-shaped valley. The fjord, being inundated with marine water, will contain seaweed and experience tidal changes; the lakes will be freshwater and relatively static.

Glacial deposits are widespread and create interesting landform features. Glaciers deposit till, an unconsolidated mixture of clay, sand, silt, pebbles, cobbles, and boulders. Depending on the area over which the glacier moved, the relative abundance of these components will vary; some till can be very sandy, while other till can be clayey. Abrasion during ice transport causes the particles in till to be usually angular in shape. Because it is the glacial ice itself that deposits this material, the till is unsorted. This lack of sorting results because the materials in till are let down to the surface as the ice melts in the same relative positions they had in the ice. Till can be formed into various morainal landforms, depending on how the glacial ice moved. Till can be deposited as ground moraine as the ice retreats. It is laid down under the ice as the ice moves along, resulting in a landscape called a till plain, which usually takes on the appearance of gently rolling, hummocky hills.

When a glacier front becomes stationary, material will continue to be moved forward through the internally forward-moving ice and be deposited along the front of the stationary glacier. It will build up into a ridge called an end moraine. If the glacier advances, the end moraine will be destroyed by the moving ice. If the glacier retreats, the end moraine will become part of the landscape. The end moraine created at the maximum forward position of the ice is called the terminal moraine. Any end moraines that form as the ice periodically retreats are called recessional moraines and usually form concentric bands of ridges in the landscape. If a glacier advances and then retreats without spending significant time in a stationary position, end moraines cannot form; the till will be formed into ground moraine.

Two types of moraines are unique to mountain glaciers. Because these glaciers are restricted to valleys, accumulations of debris tend to gather along the valley walls at the sides of the glaciers. These are called lateral moraines and are left behind as long ridges along the valley walls as the ice melts away. A medial moraine forms when two glaciers join, and their lateral moraines are consequently trapped between the two ice rivers, which are now flowing along as one glacier. Several medial moraines can form as more and more glaciers from tributary valleys join, giving the glacier a striped appearance.

Meltwater

Glacial meltwater is an important agent in creating several landform features. The power of glacial meltwater can be extraordinary, often carving deep gorges. The Channeled Scablands of Washington attest to the effects of glacial meltwaters. When glaciers melt, they release not only water but also the huge amounts of debris they carry. The material deposited by glacial meltwater is called stratified drift and differs from till in that it is sorted and layered. The heavier particles drop out first, followed by progressively smaller particles as the water travels farther from the ice, gradually slowing down. Fine clays and silts are usually transported the farthest, sometimes completely out of the area, giving the stratified drift its distinctive sand and gravel characteristics. As the glaciers eventually release less and less water, smaller and smaller particles are deposited over the coarser material deposited previously. Unlike the ice-deposited till, transport in water tends to round the particles of stratified drift.

Outwash plains are huge, flat areas of stratified drift located adjacent to where glaciers existed. If outwash is confined to a river valley, it is called a valley train. Large outwash areas are usually devoid of vegetation cover at first and are subjected to strong winds. Often, silt-sized particles in the outwash are transported by wind and deposited in thick layers downwind from the deposit. Windblown silt is called loess and is an important ingredient in soils. Soils developed on loess-capped till plains are particularly good for agriculture.

Meltwater sands and gravels can form interesting landscape features if conditions of the melting glacier are just right. If the area near the snout of the glacier becomes too thin, the ice will cease to move internally and become stagnant. When that happens, the stagnant ice may be eroded by its own meltwater and that coming off the active ice. Meltwater streams may form on the ice and drill tunnels in and under the ice. The movement of the streams determines which type of landform feature will evolve from the deposition of the sands and gravels transported by the stream. For example, if streams in or on the ice cascade down fairly vertical shafts (called moulins) in the ice or tumble down a fairly steep face of the ice edge, the sand and gravel will spill into a conical pile called a kame, which can be more than sixty meters high. Usually, several kames are formed in the same area, dotting the landscape as isolated hills.

Another example of meltwater stream deposition is eskers, which also form in zones of stagnant ice where sand and gravel are deposited within stream tunnels that are in and under the stagnant ice. When the ice melts away, long, sinuous ridges are left in the landscape. New England has some impressive eskers that are more than forty meters high in places and wind for miles through the countryside.

Other features commonly associated with outwash and stagnant ice areas are kettles, which form when chunks of ice break off from the glacier and become buried in outwash or in till as the glacier melts. Gradually, the ice chunks melt, leaving depressions in the ground, sometimes in the shape of pioneers’ kettles, which is how the name originated. Kettles that intersect the groundwater table or collect precipitation contain water and are called kettle lakes, some of which can cover several acres. An outwash area that contains numerous kettles is called pitted outwash.

Other Landforms

Actively moving ice can form streamlined shapes through either erosion or deposition. A roche moutonnée is an eroded bedrock form with a gentle up-ice side and a steep, rough down-ice side. Hence, ice-flow direction can be determined by the orientation of roches moutonnées. These features vary in size from one meter to large hills in the landscape.

Drumlins are streamlined hills that look like half-buried whales or inverted spoons composed of various materials. The cores can be composed of bedrock or, more commonly, stratified drift and are covered with a veneer of till. The formation process of drumlins is unclear, but agreement on some aspects does exist. They appear to have been formed by actively moving ice near, but not at, the edges of the ice. Drumlins always appear in groups, called drumlin fields, composed of thousands of drumlins. The most famous drumlin fields are in New York, Wisconsin, and New England.

The Great Lakes exist because of the action of continental ice sheets in the Midwest. Tongues of ice gouged out old river valleys, creating the lake basins. Meltwater and precipitation filled the basins. The history of the glacial Great Lakes from the time of the ice sheets to the present day is extremely complicated. As the ice retreated and made minor advances in various parts of the upper Midwest, different sizes and shapes of lakes emerged in each basin. The lakes had different outlets at different times, creating different water levels. Most of the old lake levels can be traced today because old beach and dune deposits exist at various elevations along the present-day Great Lakes shorelines. The old clay-rich lake beds that cover large parts of Ohio, Indiana, and Illinois have some of the richest soils in the United States.

Study of Glacial Landforms

Glacial landforms are studied using various methods. The general forms of the features are analyzed using topographic maps and various types of aerial photographs. Such tools not only illustrate the forms of each glacial feature but also show its relationship to other nearby features. Patterns in the landscape can be observed quite easily, and ice-direction movement may be interpreted. Topographic maps are very detailed, showing all natural and human-made features in a small area. These maps also include contour lines that represent elevations above sea level. With some practice, hills and valleys can be identified. All landforms large enough to be incorporated by the given contour interval can be shown on the map. All kinds of glacial features can be identified using topographic maps because their shapes are distinctively outlined by the contours.

Aerial photographs can be used in the same way as topographic maps, butt with photographs, the actual feature in the landscape is visible. Several types of aerial photographs are used for analysis depending on which features are being studied. Standard black-and-white, color, and various other wavelength bands (from satellite imagery) each emphasize distinct characteristics of the landscape.

Recall that a diagnostic characteristic for many glacial landforms is their composition. The composition of any particular feature is determined by digging into it. Sometimes, quarries or roadcuts have already exposed the interior of the structure, making the job of the geologist easier. At other locations, core samples are obtained by drilling. Sometimes, well records from existing water wells may be utilized to determine which kind of material has been deposited in an area. Another clue to the composition of a feature may be obtained by the type of soil that has developed on the material. County soil surveys are useful for this kind of analysis.

On a more detailed scale, geologists may want to understand the mechanics of ice flow when determining the creation of the landform, the direction of ice flow, the source area of the material, or the period in which the ice existed in a particular area. These pieces of information can be obtained by analyzing samples taken from the landscape features. Till fabric, pebble lithology and roundness, particle size, radiocarbon dating of organic material, and the amount and type of weathering are all commonly used observations.

Many areas rely on glaciers as a major water resource for general consumption or power generation. Mountain glaciers are carefully monitored to determine the amount of water that can be expected from them each year. The monitoring is done by either digging into the ice at various points and measuring ice layers and rates of melting or, more commonly, by taking precipitation and meltwater stream gauging measurements throughout the entire glacial basin.

Principal Terms

arête: an extremely narrow mountain ridge created between adjacent U-shaped valleys

cirque: a bowl-shaped depression near mountaintops where glaciers originate

drumlin: a streamlined hill formed under actively moving ice

esker: a sinuous ridge of stratified drift formed in a tunnel under the ice

kame: a conical hill of stratified drift

kettle: a depression created by the melting of a chunk of ice

moraine: landscapes of till, varying from fairly flat terrain to gently rolling hills to long ridges

stratified drift: material deposited by glacial meltwaters; the water separates the material according to size, creating layers

till: a mixture of unsorted, unconsolidated materials deposited by glacial ice

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