Glacial erosion shapes the landscape with landforms such as U-shaped valleys, horns, moraine and more. Two glacial cirques often combine forces, leaving an arete between them that eventually wears away over time.
Erosion rates depend on conditions and can be restricted by factors like adverse bed slopes or backpressure from terminal water bodies. Ice thickness also plays a part.
Cirque Stairways
As glaciers move down mountain valleys, they erode the surrounding terrain through processes called abrasion and plucking. Abrasion involves scraping away or gouging at rock surfaces while plucking involves breaking apart rocks to incorporate them into flowing ice flows.
Glacier velocity determines its erosion rate; however, fluctuations in subglacial water pressure and contact area have an enormous influence over abrasion and quarrying rates (Hallet, Reference Hallet1979).
One of the park’s most striking features is a circular amphitheater known as a cirque that was formed by glacier action and often contains glacial lakes known as tarns. Another type of lake found behind dams of resistant layers of rock called hanging tributary valley lakes. Examples include Swiftcurrent, Red Rock and Many Glacier Hotel lakes.
Valleys
A valley is an area of land formed by glaciers. They leave U-shaped valleys behind that may contain ribbon lakes or even be lined by moraine.
Glaciers typically erode valley floors into parabolic depressions as their discharge of ice decreases, creating an equilibrium line altitude (ELA) along its pathway. Up valley, this depression may flatten as long-term average ice discharge decreases.
Erosion of bedrocks is determined by both lithology and hydrology, in addition to slope. Till that is thin enough to deform can reduce bedrock erosion by buffering stress concentrations, damping water-pressure fluctuations, and spreading glacier force across a larger area.
Horns
Glaciers can sculpt unique landforms in their path by eroding landscapes around them, including aretes, horns and cirques.
Glaciers erode rock and soil through two primary processes – pluckeding (tearing) and dragging (dragging). These two techniques determine glacial erosion patterns, creating features such as fjords, U-shaped valleys and horns.
When multiple glaciers erode one mountain from different directions, a striking pointed peak known as a “horn” appears. You can spot these striking mountain peaks throughout hilly regions such as Switzerland and Italy or Disneyland California.
Nunataks are rocky outcrops found at the junctions between glaciers, providing indicators of past glacial extent. Additionally, their surfaces often appear smoothed out from erosion by glaciers, giving it a mountainous shape similar to a roche moutonnee.
Moraine
Glaciers scrape and cut across rocks they encounter, leaving behind patterns of long scratches called glacial striations marks that serve as indicators that an area once lay beneath their ice sheet and provide geologists with important clues as to what types of rocks exist there.
Moraines are mounds of rocks and sediment created when glaciers push their way across land, pushing up mounds of earth that become mounds after they converge at their snouts and molding into ridges at their mouths. Moraines play an integral part of landscape history; their shape providing an excellent snapshot into glacial history.
Moraines are typically made up of an intricate mixture of rock and sediment. Their main feature is their mixture of angular lithic fragments with sharp corners that display facets and striations; sediment often includes silt- and clay-sized rock flour that make up this hybrid material. A glacier’s snout provides an optimal environment to find such an environment; here the sliding rate can be optimized.
Glacial Striations
As a glacier erodes bedrock, it leaves long scratches known as striae in its path, providing valuable evidence of where its flow direction and path lie.
Glacier ice is filled with bits of rock, sediment and debris known as till that acts like coarse sandpaper to drag across bedrock surfaces and grind away at it – creating patterns known as glacial striations in their wake.
Striations mark the direction of ice flow and can be used to identify its ‘provenance’; this information helps determine the size and velocity of glaciers; additionally, their maps help estimate their estimated speed – with faster glaciers leading to more erosion and consequently faster striation maps.