14.5: Glacial Landforms
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Glacial landforms are of two kinds, erosional and depositional landforms. Erosional landforms are formed by removing material. The internal pressure and movement within glacial ice cause some melting and glaciers to slide over bedrock on a thin film of water. Glacial ice also contains a large amount of sediments such as sand, gravel, and boulders. Together, the movement plucks off bedrock and grinds the bedrock producing a polished surface and fine sediment called rock flour as well as other poorly-sorted sediments. Characteristic depositional landscapes are produced when the ice melts and retreats and leaves behind these sediments with distinct shapes and compositions. Because glaciers are among the earliest geological processes studied by geologists whose studies were in Europe, the terminology applied to glaciers and glacial features contains many terms from European languages.
Erosional Glacial Landforms
Both alpine and continental glaciers erode bedrock and create erosional landforms. Glaciers are heavy masses of abrasive ice that grind over the surface. Elongated grooves are created by fragments of rock embedded in the ice at the base of a glacier scraping along the bedrock surface called glacial striations. In addition, rock flour as fine grit in the ice can polish a hard granite or quartzite bedrock to a smooth surface called glacial polish. This figure illustrates these glacial landforms.
Following is a description of erosional landforms produced by alpine glaciers. Since glaciers are typically much wider than rivers of similar length, and since they tend to erode more at their bases than their sides, they transform former V-shaped stream valleys into broad valleys that have relatively flat bottoms and steep sides with a distinctive “U” shape . Little Cottonwood Canyon near Salt Lake City, Utah was occupied by a large glacier that extended down to the mouth of the canyon and into Salt Lake Valley . Today, that canyon is the location of many erosional landforms including the U-shaped valley, as well as polished and striated rock surfaces. In contrast, river-carved canyons have a V-shaped profile when viewed in cross-section. Big Cottonwood Canyon, its neighbor to the north, has that V-shape in its lower parts, indicating that its glacier did not extend clear to its mouth but was confined to its upperparts.
When two U-shaped valleys are adjacent to each other, the ridge between them can be carved into a sharp ridge called an arête. Since glaciers erode a broad valley, the arêtes are left behind with steep walls separating them. At the head of a glacially carved valley is a bowl-shaped feature called a cirque representing where the head of the glacier is eroding against the mountain by plucking rock away from it and the weight of the thick ice is eroding out a bowl. After the glacier is gone, the bowl at the bottom of the cirque is often occupied by a lake called a tarn. Headward cirque erosion by three or more mountain glaciers produces horns, which are steep-sided, spire-shaped mountains with pronounced cirques on three or more sides. Low points along arêtes or between horns (also mountain passes) are termed cols. When a smaller tributary glacier intersects a larger trunk glacier, the smaller glacier erodes down less. Therefore, once the ice has been removed, the tributary valley is left as a hanging valley, sometimes with a waterfall. Where the trunk glacier straightens and widens the former V-shaped valley and erodes the ends of side ridges, the result is a steep triangle-shaped cliff called a truncated spur.
|Stage Number||Stage Name||Description|
|I||Grooved Upland||Early-stage. Only cirques (C) developed at this point.|
|II||Early-Fretted Upland||Cirque (C) development moves headward, very much like a stream and headward erosion. Still no ridge-like glacial features.|
|III||Mature-Fretted Upland||Classic mountain glacial landscape. Horns (H) and Arêtes (A) have developed.|
|IV||Monumented Upland||Final stage of mountain glacial erosion. Erosion so extensive that monuments (M) of the mountain and knife-like ridges are some of the only features remaining.|
Depositional Glacial Landforms
Sediment is deposited by glaciers in both alpine and continental environments. Since ice is responsible for a large amount of erosion, there is a lot of sediment in glacial ice. When sediment is left behind by a melting glacier, it is called till and is characteristically poorly sorted with grain sizes ranging from clay and silt to subrounded pebbles and boulders, possibly striated. Many of the depositional landforms described in this section are composed of till. Lithified rocks of this type are sometimes referred to as tillites, though that implies an interpretation of glacial origin. A more objective and descriptive term is diamictite, meaning a rock with a wide range of clast sizes.
Material carried by the glacier is called moraine, which is an accumulation of glacial till produced by the grinding and erosive effects of a glacier. In valley glaciers, moraine also includes material falling on the sides of the glacier by mass wasting from the valley walls. The glacier acts like a conveyor belt, carrying sediment inside and on the ice and depositing it at the end and sides of the glacier. Because ice in the glacier is always flowing downslope, the deposited moraines build up even if the end of the glacier doesn’t advance. The type of moraine depends on its location. A terminal moraine is a ridge of unsorted till at the maximum extent of a glacier or the farthest extent of a glacier . When glaciers retreat in episodes, they may leave behind deposits called recessional moraines. The recessional moraines look similar to terminal moraines but are formed when the glacier retreat pauses. Moraines located along the side of a glacier are called lateral moraines and mostly represent material that fell on the sides of the glacier from the mass wasting of the valley walls. When two tributary glaciers join together, the two lateral moraines combine to form a medial moraine. Behind the terminal and recessional moraines is a veneer (or thin sheet) of till on top of bedrock called the till sheet (or ground moraine).
In addition to moraines, as glaciers melt they leave behind other depositional landforms. The intense grinding process creates a lot of silt. Streams of water melting from the glacier carry this silt (along with sand and gravel) and deposit it in front of the glacier in an area called an outwash plain. In addition, when glaciers retreat, they may leave behind large boulders of a type of rock that doesn’t match the local bedrock. These are called glacial erratics. When continental glaciers melt, large blocks of ice can be left behind to melt within the impermeable till and can create a depression called a kettle that can be later filled with surface water like a kettle lake. As glaciers melt, the meltwater flows over the ice surface until it descends into crevasses, perhaps finding channels within the ice or continuing to the base of the glacier into channels along the bottom. Such streams located under continental glaciers carry sediment in a sinuous channel within or under the ice, similar to a river. When the ice recedes, the sediment remains as a long sinuous ridge known as an esker. Meltwater descending down through the ice or along the margins of the ice may deposit mounds of sediment that remain as hills called kames.
Also common in continental glacial areas of New York state and Wisconsin are drumlins. A drumlin is an elongated asymmetrical drop-shaped hill with its steepest side pointing upstream to the flow of ice and streamlined side (low angle side) pointing in the direction the ice is flowing.
Drumlins can occur in great numbers in drumlin fields. The origin of drumlins is still debated and some leading ideas are the incremental accumulation of till under the glacier, large catastrophic meltwater floods located under the glacier, and surface deformation by the weight of the overlying glacial ice [12; 13].
|1||Receding Glacier||Sheet of ice with a terminus that is moving back with time.|
Lakes are common features in alpine glacial environments. A lake that is confined to a glacial cirque is known as a tarn such as Silver Lake near Brighton Ski resort located in Big Cottonwood Canyon or Avalanche Lake in Glacier National Park. Tarns are common in areas of alpine glaciation because the thick ice that carves out a cirque also typically hollows out a depression in bedrock that after the glacier is gone, fills with precipitated water.
In some cases, recessional moraines may isolate a series of basins within a glaciated valley, and the resulting chain of lakes is called paternoster lakes.
Lakes that occupy long glacially carved depressions are known as finger lakes .
In areas of continental glaciation, the crust is depressed isostatically by crustal loading from the weight of thick glacial ice. Basins are formed along the edges of continental glaciers (except for those that cover entire continents like Antarctica and Greenland), and these basins fill with glacial meltwater forming proglacial lakes. The classic example of a proglacial lake is Lake Agassiz, located mostly in Manitoba, Canada, with Lake Winnipeg serving as the remnant of the lake. Many such lakes, some of them huge, existed at various times along the southern edge of the Laurentide Ice Sheet.
Other proglacial lakes formed when glaciers dammed rivers causing the valley to be flooded. The classic example is Glacial Lake Missoula, which formed in Idaho and Montana when the Clark Fork River was dammed by the ice sheet. During the latter part of the last glaciation about 18,000 years ago, the ice holding back Lake Missoula was breached enough to allow some of the lake water to start flowing out, which escalated into a massive and rapid outflow (over days to weeks) during which much of the volume of the lake drained along the valley of the Columbia River to the Pacific Ocean. The ice dam and the lake then formed again. It is estimated that this process of massive flooding happened at least 25 times over that span. In many cases, the rate of outflow was equivalent to the discharge of all of Earth’s current rivers combined.
The landscape produced by these massive floods is preserved in the Channelled Scablands of Idaho, Washington, and Oregon .
During the last glaciation, most of the western United States had a cooler and wetter climate. Due to less evaporation and more precipitation, many large lakes formed in the basins of the Basin and Range Province called pluvial lakes. Two of the largest pluvial lakes were Lake Bonneville and Lake Lahontan. Lake Bonneville occupied much of western Utah and eastern Nevada (Figure from Miller et al 2013) and filled Salt Lake Valley, which is densely urbanized today, with water hundreds of feet deep. During the last glaciation, the level of the lake fluctuated many times leaving several pronounced old shorelines in the western portion of Utah including Salt Lake Valley identified by wave-cut terraces. Lake level peaked around 18,000 years ago  and spilled over Red Rock Pass in Idaho into the Snake River causing rapid erosion and a very large flood that rapidly lowered the lake level and scoured the land in Pocatello Valley, Snake River Plain, and Twin Falls Idaho. The flood ultimately flowed into the Columbia River across part of the scablands area and had an incredible discharge of about 4,750 km3  which is about the volume of Lake Michigan. This would be as if one of the Great Lakes completely emptied within days. Lake Lahontan existed at about the same time mostly in northwestern Nevada.
The five great lakes in the upper Midwest of North America occupy five basins carved by the ice sheet in a large depression during the Ice Age and were exposed as the ice retreated about 14,000 years ago. They form a naturally interconnected body of freshwater that drains into the Atlantic through the St. Lawrence River. Emergent coastline features are forming on the lakes due to isostatic rebound since the ice retreated (see Chapter 12).
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