13.3: Landforms formed by glacial erosion

Bergschrund 

A glacier actively takes part in erosional activities that are performed as a result of hydraulic plucking and abrasion. Here we will look at some landforms created primarily by glacial erosion. 

Bergschrund is formed in Alpine Glaciers. As shown in Figure \(\PageIndex{1}\), it is a deep crevasse or multiple crevasses that are formed at the head of a glacier where the glacier separates from the steep rock face or ice attached to a cirque's back wall. A common reason for the formation of a bergschrund is differential movement of ice. Gravity makes the main mass of ice move downstream. Some of the ice clings on to the steepest part of the mountain headwall and is either immobile or moves very slowly. Stress is created as the fast moving ice pulls away from the stagnant upper ice resulting in the opening up of tensional cracks. Bergschrund are geologically very significant as meltwater enters through these cracks, the freeze-thawing results in frost shattering and plucking of the rock face results in eventual formation of a corrie. 

Figure \(\PageIndex{1}\): Bergschrund in Mt Dolent which is a part of Mt Blanc massif in the border of France, Italy and Switzerland. (CC BY-SA 4.0; Tikgeit via Wikimedia Commons).

Cirque or Corrie 

As the glacier moves downstream from its snow-covered valley head, intense shattering of the upstream slopes produce a depression where granular ice accumulates. Hydraulic plucking of the backwall steepens it while the floor is abraded as the ice moves making it deeper. As a result of these erosional processes an arm-chair like depression is created. Commonly there is a rocky ridge at the exit of the cirque where ice eventually melts and creates a corrie lake or tarn. Figure \(\PageIndex{2}\) diagrammatically represents the formation of a cirque. Cirque Peak is found in the Canadian Rockies, west of Dolomite Pass in the Bow River valley is an example of a cirque. 

Figure \(\PageIndex{2}\): Formation of a cirque. (CC BY-SA 3.0; Clem Rutter and Rochester, via Wikimedia Commons).

Figure \(\PageIndex{2}\) is a cross-sectional view of a cirque glacier occupying a bowl-shaped mountain depression. It illustrates the major erosional processes and landforms associated with alpine glaciers.

At the left side of the figure is a steep rock face labeled Headwall. Between the headwall and the upper surface of the glacier is a large crevasse labeled Randkluft (Bergschrund), a gap that forms where the moving glacier separates from the stationary rock wall.

Near the headwall is a region labeled Zone of Plucking. In this area, ice freezes into cracks in the rock, loosening and removing rock fragments as the glacier moves. This process steepens the headwall and enlarges the cirque basin.

The glacier occupies the center of the diagram and is labeled Glacial Ice. A large curved arrow within the ice indicates downslope movement of the glacier from the headwall toward its terminus.

At the base of the cirque basin is an over deepened depression. A note indicates that a tarn (lake) will form here when the glacier melts. Tarn lakes commonly occupy basins excavated by glacial erosion after the ice retreats.

Toward the lower right side is a region labeled Zone of Abrasion. As the glacier moves, rock fragments embedded in the ice scrape and grind against the underlying bedrock, smoothing and deepening the valley floor.

At the far right end of the glacier is a pile of rock debris labeled Terminal Moraine. This accumulation marks the furthest extent of the glacier and consists of sediment transported and deposited by the ice.

The figure shows how a cirque glacier modifies the landscape through the combined processes of plucking and abrasion. Plucking erodes and steepens the headwall, while abrasion deepens and smooths the basin floor. The glacier transports sediment downslope and deposits it as a terminal moraine. After glacial retreat, the over deepened basin may fill with water to form a tarn lake. Together, these processes create the characteristic landforms of alpine glaciation.

Arete and Pyramidal Peak

Glacial erosion of a mountain creates distinctive features through headward erosion. When two adjacent cirques cut back on opposite sides of the mountain, ridges are formed called aretes. When three or more aretes cut back on each other an angular horn is formed called a pyramidal peak. Figure \(\PageIndex{3}\) illustrates cirque, arete and pyramidal peak on Mt. Connes in the Yosemite Valley. 

Figure \(\PageIndex{3}\): Cirque, arete and pyramidal peak. (Public Domain; Gary Stockbridge via Picryl).

Figure \(\PageIndex{3}\) contains two panels comparing alpine glacial landforms.

Panel A: Schematic diagram of alpine glacial features - The left panel is a labeled drawing showing landforms created by mountain glaciers. Key features include:

• Horn – a sharp, pyramidal peak formed where several cirques erode into a mountain from different sides.
• Arête – a narrow, knife-edged ridge separating adjacent cirques or glacial valleys.
• Cirque – a bowl-shaped depression carved into a mountainside by a glacier.
• Tarn – a small lake occupying a cirque after the glacier has melted.
• Col – a low saddle or pass formed between neighboring cirques.
• Hanging Valley – a tributary glacial valley left elevated above the main valley after glaciation.
• Combe – a steep-sided hollow or depression associated with glacial erosion.
• Glacial Step – an abrupt change in valley floor elevation created by differential glacial erosion.
• U-shaped Trunk Valley – a broad, steep-sided valley carved by the main glacier.

Together, these features illustrate how glaciers reshape mountainous terrain through erosion.

Panel B: Glacial features on Mount Conness - The right panel is a photograph of a glaciated mountain landscape with the same features labeled in a real-world setting. Visible features include:

• A horn at the highest peak.
• An arête forming sharp ridges.
• A cirque containing a small glacier.
• The remaining glacier occupying part of the cirque.
• A tarn lake at the valley floor.
• Lateral moraine deposits along the sides of the former glacier.
• A terminal moraine marking the furthest extent of glacier advance.

The figure demonstrates how alpine glaciers modify mountain landscapes through erosion and deposition. Glacial erosion creates cirques, arêtes, horns, hanging valleys, and U-shaped valleys, while glacial deposition forms moraines. After glaciers retreat, depressions carved by the ice often fill with water to form tarns. The photograph confirms how these theoretical landforms appear in an actual alpine environment.

U-shaped Glacial Trough 

As a glacier moves downward, it is fed by ice from several corries (similar to the way tributaries feed a river) and it begins to erode the sides and floor of the valley it is moving through. It abrades and grinds the bedrock and straightens protruding spurs on its path. The interlocking spurs are eroded to form truncated spurs and the valley floor is deepened. The valley takes an U shape with a wide flat floor and steep sides. After the glacier retreats or the ice melts, the valley floor may be covered by ribbon lakes . Example of a Ribbon lake is Peyto Lake in the Banff National Park, Canada shown in Figure \(\PageIndex{4}\). Peyto Lake is formed by the Peyto Glacier. 

Figure \(\PageIndex{4}\): Peyto Lake in Banff, Canada. U-shaped glacial valley is photographed here. (CC BY-NC 4.0; Mrinmoy Ghosh).

Hanging Valley 

Main Glacier has a bigger glacier with more mass and erodes its valley more than the tributary glaciers. After the ice melts, the tributary glacial valleys hang above the main glacial valley. It is common to see meltwater coming down as waterfalls from the tributary glaciers to the main glacial valleys. The tributary glacial valleys which are at a higher elevation are called hanging valleys. In Yosemite National Park, a hanging valley is found where the Bridalveil Fall falls to the valley floor from higher elevation. 

Glaciers in adjacent tributary valleys scour away at the upland between them (an interfluve). As erosion of the interfluve continues, it narrows into a serrated ridge known as an arête. As mountain glaciers flow down valley they encounter exposed bed rock of varying resistance to erosion. The glacier will erode down into weaker rock but have to flow over the stronger rock. This creates a series of rock steps composed of the more resistant rocks with small depression behind them where the weaker rock is exposed. Often these depressions are occupied with water to form staircase lakes. Connected by a small stream, these lakes are also known as paternoster lakes.

Fjord 

As a glacier forms and flows through a pre existing river valley in a mountainous region, it enhances the valley making it deeper and wider by the glacial processes of abrasion and plucking. The glacier transforms the glacial V shaped valley into a U shaped. As a result of global warming as sea level rises, it floods the U-shaped valley and its deepest parts are inundated by the sea or ocean. The land that was depressed by the weight of the glacier rises slowly once the glacier retreats. The flooded U-shaped valley becomes a fjord. Often there is a sill at the entrance to a fjord formed by a moraine. Some examples of fjords are the Geirangerfjord and Sognefjord in Norway and Scoresby Sund in Greenland. Figure \(\PageIndex{5}\) is an aerial photograph of a large fjord system in Scoresby Sund, Greenland, showing a deep, dark blue body of water surrounded by towering, steep-walled, snow-capped cliffs. Large icebergs are visible scattered throughout the water.

Figure \(\PageIndex{5}\): Scoresby Sund, Greenland. (CC BY 2.0; Anttilipponen via Wikimedia Commons).

Roche Moutonnee

A roche moutonnee is an asymmetrical resistant rock hummock which is striated due to ice movement. As illustrated in Figure \(\PageIndex{6}\), the upstream side is smoothened by abrasion and the downstream side is steeper and roughened by plucking. In French, the name means "sheep backed rock" to resemble the way it looks. This feature's long axis is aligned with the direction of ice flow so it provides an evidence of the path of ice movement. The Cairngorm Mountains of Scotland are an example of Roche Moutonnee. 

Figure \(\PageIndex{6}\): Roche Moutonnee. (Public Domain; National Parks Gallery via Picryl).

Figure \(\PageIndex{6}\) illustrates the formation of a roche moutonnée, an asymmetrical bedrock hill shaped by glacial erosion.

A glacier moves from left to right, indicated by a large arrow labeled Direction of glacial flow. The glacier flows over a mass of jointed bedrock protruding above the surrounding surface.

The upstream side of the hill is labeled the stoss side. This side has a smooth, gently sloping surface where the glacier presses against the bedrock. As rock fragments embedded in the ice move across the surface, they grind and polish the rock through abrasion.

The downstream side is labeled the lee side. This side is steeper and more irregular. As the glacier passes over the crest of the rock, pressure decreases, allowing meltwater to refreeze in fractures. This process breaks loose blocks of rock that are removed by the ice through glacial plucking. Rock fragments are shown accumulating and being carried away on the lee side.

The glacier itself is shown flowing over the bedrock obstacle, with arrows indicating ice movement across the stoss side, over the crest, and down the lee side.

Crag and tail 

Crag and tail, illustrated in Figure \(\PageIndex{7}\) is a glacial landform that is created by both glacial erosion and deposition. When there is a resistant rock outcrop called crag in the path of a glacier, the rock resists the erosive powers of the moving glacier. As the glacier flows over the rock, the upstream side gets eroded by abrasion and plucking. A craggy surface is created as a result of these erosive forces. The resistant rock acts as a obstacle forcing the ice the move over and around it and protects the softer rock behind it. The immediate down-glacier side is protected from erosion and the glacier deposits till in a long, tapering, gently sloped ridge called tail which points in the direction of glacial movement. The Edinburgh Castle in Scotland is built on a resistant volcanic crag.

Figure \(\PageIndex{7}\): A diagram of a crag and tail feature. A:Crag of hard volcanic rock, B:Tail of softer rock, C:Direction of ice movement. (CC BY-SA 3.0; Jonathan Oldenbuck via Wikimedia Commons).

Figure \(\PageIndex{7}\) illustrates the formation of a crag and tail, a landform created by glacier movement across an area containing resistant rock.

A glacier, represented by the dashed line and arrow labeled C, moves from left to right across the landscape. In its path is a large mass of resistant rock labeled A, which forms the crag.

The resistant rock obstructs the glacier's flow and protects the area immediately behind it from erosion. As a result, a wedge-shaped deposit labeled B develops on the downstream side of the crag. This feature is called the tail. The tail consists of softer material that remains preserved because it lies in the sheltered zone behind the resistant rock.

The steep side of the landform faces the direction from which the glacier approached, while the elongated tail extends in the same direction as glacier movement.