Glacial erosion

James S. Aber

Table of Contents
Introduction Fundamentals
Mechanisms Erosional forms
Alpine glaciation References

Introduction to geomorphology

Geomorphology is the study of the Earth's surface landforms both on land and on the sea floor. This study is both descriptive and quantitative; it deals with morphology, processes, and origins of landforms. The ultimate goals of geomorphology are to understand the way in which landforms are created and to document the evolution of landforms through time. The geomorphology of any region or site is the result of interplay involving three factors:

  1. Structure: refers to the nature of solid material that forms the surface, its composition, texture, fabric, architecture, mechanical strength, and other physical attributes.

  2. Process: refers to the physical, chemical, or biological processes that shape the surface into landforms. Broadly speaking, processes are either depositional (constructive) or erosional (destructive).

  3. Time: refers both to the rate at which a process modifies the surface and to the length of time or duration that a process has operated at a site.

All land surfaces are subject to diverse processes that operate at greatly varying rates. Static landscapes do not exist; all landscapes undergo constant modification. The active processes also change through time, so that every landscape is subject to continual evolution.

The processes that shape landforms can be categorized as endogenetic or exogenetic. Endogenetic processes are related to plate tectonics and to the surface effects of plate movements, both horizontally and vertically, as well as to other processes originating from the Earth's interior. Exogenetic processes develop at or above the surface in the atmosphere, hydrosphere, cryosphere, or biosphere. They involve wind, water, ice, mass movements, or living organisms that modify landforms. Geomorphic processes associated with glaciation are among the most variable and complex of any environments at the Earth's surface.

Endogenetic and exogenetic processes combine with structure and time to produce the observed landforms at the Earth's surface. Most landforms involve a considerable mass of material, and so are slow to adapt when environmental changes take place. The geomorphology of a region, therefore, represents a long-term integration of environmental conditions and trends. A region's geomorphology is, thus, a reflection of both past and present environments.

Fundamental effects of glaciers

The fundamental geomorphic effects of glaciers and ice sheets on the landscape are threefold: erosion, deformation, and deposition. At least two of these three categories of features must be observed in order to demonstrate former glaciation of a region. In order to interpret ancient glaciation, the uniformitarian approach is crucial: modern glaciers and their erosional and depositional features are used as analogs for understanding similar features of older glaciations.

Erosion or deposition beneath a glacier result from a complex relationship between ice velocity, basal melting/freezing, and substratum conditions. In general, erosion takes place under most of the accumulation zone, whereas deposition predominates in the ablation zone. The greatest amount of erosion takes place where ice velocity is highest, immediately inside the equilibrium line or in concentrated flow of ice streams.

Glacial landforms comprise many kinds of features created by glacial destruction and/or construction. Destructive landforms are the results of erosion or deformation in which material is removed from the local landscape. Constructive landforms are created by building up material due to deposition or deformation. Both glacier ice and glacial melt water are active agents of glacial landform genesis.

Mechanisms of glacial erosion

Glacial erosion occurs in several ways. Abrasion is the result of stones and grit in the basal ice grinding over the substratum. The products of grinding are rock flour sediment and a variety of erosional bed forms: polish, striations, grooves, fractures, and gouges. This mechanism implies basal sliding and, thus, a temperate thermal regime with water beneath the glacier. This type of erosion is enhanced by extensional (downward) ice flow conditions.

Large blocks of the substratum may be removed by a process called plucking or quarrying. This takes place when blocks of the substratum become frozen onto the ice base and are lifted up into the glacier. Freezing on may occur where ice is advancing from thawed-bed to frozen-bed zones either locally or regionally, and the removal of sediment is facilitated by compressive (upward) ice flow.

Subglacial melt water is another important agent of erosion. High-pressure water charged with sediment may be highly erosive, and can carve potholes and distinctive hydraulic channel forms. Sudden outbursts or floods of subglacial melt water, may remove huge volumes of sediment from beneath the glacier very rapidly (Booth 1994). It should be emphasized that all mechanisms of glacial erosion may operate in close association or may alternate through time at a particular site.

Glacial polish on granite surface, island of Bornholm, Denmark. Subglacial grinding produced this polished surface. Ice movement from left to right; scale pole marked in 20-cm intervals.
Giant glacial groove on Kelleys Island, Ohio. Many giant grooves were once present on Kelleys Island. Photo shows about half of a preserved groove.
Glacial grooves on Kelleys Island are cut into Paleozoic dolostone. The grooves were 2-6 m deep, 5-20 m wide, and 100-400 m long (Goldthwait 1979). This view depicts intricate striations and small grooves within the main channel.
Crescentic gouges on Sioux Quartzite, southwestern Minnesota. Such gouges are produced by ice pressing and dragging a boulder over the bedrock surface. Ice movement in direction of needle (35 cm long); gouges are convex in the downice direction.
Crescentic gouges, shallow grooves and striations on Sioux Quartzite, southwestern Minnesota. Ice movement in direction of needle (35 cm long); gouges are convex in the downice direction.
Small potholes on a rock knob in front of Nigardsbreen, an outlet glacier of Jostedalsbreen, western Norway. These potholes were carved in crystalline bedrock by subglacial melt water when the glacier extended over the area.

Erosional forms

Destructive (erosional) landforms range in size from small abrasion marks (mm to cm deep), to grooves and channels (m to 10s m deep), to valleys and basins (100s to 1000s m deep). Chamberlin (1886) compiled the largest and still best set of field observations concerning glacial striations and grooves. He recognized three dominant types of striations, those which:

  1. Begin shallow and narrow, and steadily increase in width and depth to a maximum, and then suddenly terminate.
  2. Begin and increase, as in the first case, reach a maximum size, and then gradually taper out.
  3. Start abruptly and then gradually taper out.

Striations and grooves are often used to establish the direction of ice movement at the time of erosion both locally and regionally. Glaciations with shifting directions of movement can create complex patterns of crossing striations. In some cases, it is possible to determine the relative age (order) of ice movements by cross-cutting erosional forms. Prolonged glacial erosion may ultimately strip large regions of all soft sedimentary cover so that vast shields are created.

Large glacial erratic displays striations and grooves as well as various types of fracture marks. Obrzycko, west-central Poland.

A characteristic kind of erosional landform is called stoss-and-lee topography. The stoss (upice) side is smoothed, striated and polished, in contrast to the lee (downice) side, which is rough and irregular. Smoothing of the stoss side is a result of pressure-melting and abrasion; the lee side is formed by regelation and plucking of angular blocks. Stoss-and-lee topography is developed at several scales: individual boulders, small bedrock knobs—called roche moutonées, and sizeable hills.

Crossing sets of striations on Sioux Quartzite, Jeffers Petroglyphs, southern Minnesota. Needle and compass mark ice movement directions, which differ by about 30°. Ice movement toward bottom of view.
Crossing crescentic gouges on Sioux Quartzite, southwestern Minnesota. Part of the upper gouge is truncated by the lower gouge; the upper gouge is older, the lower one is younger. Needles indicate directions of ice movements, which differ by about 50°.
Glaciated crystalline bedrock of the Fennoscandian shield, Stockholm, Sweden. This view is directed downice; note polished and smoothed stoss surfaces (compare with image below). Glacial grinding and abrasion took place on the stoss sides of bedrock knobs.
Glaciated crystalline bedrock. This view is in upice direction; note rugged "stairstep" form of lee surfaces (compare with image above). Plucking (quarrying) occurred on lee sides of obstacles.
View of small roche moutonée formed on Sioux Quartzite near Jeffers, southern Minnesota. View is parallel to long axis of knob, which stands about 1 m above the adjacent glacial pavement. Smoothly streamlined upice end, top, and sides are visible. Ice movement toward background of view. This bedrock knob was freshly exposed when photographed.

Alpine glaciation

Glaciated mountains display many typical forms created primarily by erosion. Ice-carved valleys display a U-shaped cross profile: steep side walls with truncated spurs and a gently rounded to flat valley floor. Hanging valleys and waterfalls are often found along the sides of deep glacial valleys. The longitudinal profile of a glacial valley usually consists of a series of basins separated by rock barriers (riegels) or moraine sills. The heads of alpine glacial valleys are often marked by ampitheater-shaped cirque basins, and where cirques converge on mountain sides, narrow arêtes (ridges) or horns (peaks) are left as residual features. Lakes are extremely common within cirques and formerly glaciated valleys, because of various rock basins and moraine dams.

Glaciated valley, near Voss, western Norway. View from head of the valley in downice direction. Note steep valley sides and gently rounded valley floor—classic U-shaped valley cross profile. Also notice highway zig-zag turns up the valley head in foreground.
Waterfalls spill from a hanging valley on the side of a deeper valley. Geisdalsfossen, western Norway.
Okanagan Lake, British Columbia. NASA space-shuttle photograph, STS068-155-011, 10/01/94, 5-inch format. Low-oblique view to south. Lake Okanagan valley is a deeply eroded glacial trough in the Canadian Rocky Mountains. NASA Johnson Space Center, Imagery Services.
Small, lake-filled cirques in Tatra Mountains, southern Poland. The upper cirque (right) is separated from the lower one by a rock barrier. Each rock basin was excavated by a small cirque glacier. For more information on the Tatras—go to Tatra Mountains.
Morskie (Lake) Oko, Tatra Mountains, southern Poland. This alpine lake is dammed by a massive end moraine, located on the far side of the lake in this view over one of Poland's most popular tourist destinations.
View of Píco Aguila, Andes Mountains, Venezuela. This steep peak is a horn created by erosion of cirque glaciers in surrounding valleys. The peak reaches more than 4000 m altitude.

Spectacular horns, arêtes, deep valleys, and small lake basins as seen from the observatory atop Lomnica, the second highest peak (2634 m) in the Tatra Mountains, Slovakia. All photos © J.S. Aber.

Views of Königssee, a lake in a deep ice-carved valley on the northern side of the Alps, Berchtesgaden national park, southern Germany.

Where montane glaciers or ice sheets descend into the sea, spectacular fjord valleys may be carved 100s to 1000s of m below sea level. The world's deepest fjords are in Antarctica: Vanderford, Vincennes Bay (2287 m) and Skelton Inlet (1933 m), and many fjords cut through the mountains of coastal Greenland. Fjord entrances are usually quite shallow with shoals and small islands; deep basins are located well inland from the mouth, as demonstrated by the classic fjords of western Norway.

The locations of fjords may be related to preglacial valleys, structural (bedrock) control, or crustal fractures (Holtedahl 1967). Fjords are the results of combined glacier erosion and erosion by high-pressure melt water flowing beneath the ice. The positions of fjord mouths mark the points where the valley glaciers either began to float or could spread out laterally, so that erosion was much less than in the confined inland valley.

Space-shuttle photograph of southern tip of Greenland. Low-oblique view toward north, color-visible, 70-mm format, 12/88. All land areas are snow covered in this mid-winter scene. Deep glacier-carved fjord valleys cut through the coastal mountain ranges. The fjords define straight paths and meet at angular junctions, which indicate that glacial erosion followed crustal fractures. NASA Johnson Space Center, Imagery Services, STS027-36-49.
View over interior portion of Hardangerfjord, western Norway. This is the deepest portion of the fjord—water depth more than 800 m, located 100 km inland from its opening to the sea. Note ship on far side of fjord for scale.
Giant "half potholes" on side of Granvin valley, a tributary to Hardangerfjord, western Norway. Sculpted side of valley was carved by high-pressure melt water flowing between the glacier and valley wall.
Closeup view of single pothole, which is carved into metamorphic bedrock, Granvin valley, Norway (see above).
Overview of Bolstadfjord, an interior fjord of western Norway. The main fjord basin is 100s of m deep in center. The fjord's entrance is located at the far end of this view (see below).
Closeup view of Bolstadfjord's mouth beneath the bridge. The entrance is narrow and quiet shallow; notice tidal current flowing out of fjord (to left).

Glossary or references.

Return to Glacial geomorphology (2020).
All images and text © J.S. Aber.