Nature’s Landscape Architects

Glaciers are mighty sculpters of the landscape. They can carve wide valleys and deposit large expanses of rolling plains of sediment. How do they accomplish these feats? What are the various landforms that they can carve or deposit?

 

How Glaciers Carve Rock

If you take a piece of ice and attempt to scratch a rock like granite or basalt, what happens? Will the ice actually succeed in scratching the rock? No. Rock is harder than ice, so it is the ice that would get scratched. So how do glaciers manage to carve out large tracks of land if ice is softer than the rock it is trying to shape?

There are two processes that glaciers use to do this seemingly impossible task. First, water gets into the cracks and joints of bedrock over which glaciers flow, then freezes. As water freezes, it expands and loosens the rock. Then a glacier can pick up the rocks and carry them away. This is known as glacial plucking.

Second, glaciers can serve as sandpaper and chisels. Sediment and rock frozen to the bottom and sides of glaciers scour, grind, and scrape the rock surfaces over which they flow, gradually wearing them down and away. This abrasion of rock against rock can scour the landscape and leave large gouges, small striations, or even a finely polished surface. In the end, the sediment the glacier uses to abrade the surface may be ground so fine it becomes glacial flour. In some cases chips may be vibrated off leaving chattermarks.

Influences of Glacial Erosion

There are many factors that influence glacial erosion.

Temperature

The difference between a warm glacier at the pressure melting point and a cold one is profound. A warm glacier is loose on its bed; a cold glacier is solidly frozen to the bed. A warm glacier continuously slips over its bed; a cold glacier slips only occasionally and locally, if at all. Hence, the glacier that slips more, that is the warm glacier, erodes more bedrock.

Amount and Nature of Debris in Basal Ice

Clean ice is not an effective agent of abrasion. Ice laden with bits of rock particles is effective. The difference is like trying to smooth a board by rubbing it with an ice cube, compared to rubbing it with sandpaper.

Water

Water at the bed of a warm glacier inhibits more than it enhances the abrasion process because it keeps small particles from making good contact with the bed. Water can be an asset to plucking, however.

Ice Thickness

If you are serious about sanding a board, you press down firmly as you rub, right? Glaciers follow the same principle, and as you might expect, abrasion is greater under thick ice. Ice thickness serves as overburden pressure.

Word of Warning: To abrade effectively, sediment and rocks must be solidly gripped in the moving ice and pressed firmly against the glacier’s bed. Pressure melting can occur, however, and cause the glacier to loose its grip on sediment and rocks. This lessens the amount of abrasion even though overburden pressure and flow velocity have increased.

Flow Velocity

Again, if you are a serious sander of boards, you will rub rapidly. Rubbing rapidly is more effective, temporarily at least, than rubbing slowly. In a similar way, velocity of ice flow is clearly a factor in effective erosion.

Word of Warning: To abrade effectively, sediment and rocks must be solidly gripped in the moving ice and pressed firmly against the glacier’s bed. Pressure melting can occur, however, and cause the glacier to loose its grip on sediment and rocks. This lessens the amount of abrasion even though overburden pressure and flow velocity have increased.

Resistance of Bed Materials

The nature of bedrock over which glaciers flow plays a significant role in erosion. Take, for example, quartzite—a hard, partly recrystallized rock composed of the mineral quartz (a 7 on the Moh’s Hardness Scale of 10). Quartzite is much less easily abraded than a bed of marble, which is composed of the mineral calcite (hardness of 3). Conversely, the quartzite may be much more susceptible to plucking because of its brittleness and tendency to be jointed. Marble, on the other hand, is not particularly brittle. Hence, one type of glacial erosion (abrasion vs. plucking) may be more effective than another depending on the bedrock over which a particular glacier flows.

Structure of Bed Materials

As agents of erosion, glaciers are adept at finding zones of weakness in rocks. Thus, glaciers exploit the structures of bed materials, such as layering in sedimentary rocks and jointing in volcanic rocks. Hence, these big, slow-moving bodies of ice can be quite subtle in seeking out “weaknesses” in rocks.

 

Features of Erosion

Here is an illustration of a landscape that has been sculpted by glaciers. Each of the yellow dots represents a glacial landform created by the erosion of the pre-existing landscape.

Arête

Sharp, usually serrate, rock ridge between two steep, glacially sculpted slopes.

Cirque

Steep-walled, gentle-floored, semicircular topographic hollow created by glacial excavation high in mountainous areas.

Col

Open, U-shaped pass across a high, narrow mountain ridge created by glacial erosion.

Fjord

Long, narrow, deep arm of the sea filling a glaciated coastal mountain valley.

Glacial Basins

One thing that glaciers do well, given favorable conditions such as thick, fast-moving, warm ice and well-jointed bedrock, is excavation. Localized excavation can create closed bedrock basins, many of which ultimately harbor lakes. Examples of these lakes are tarns and paternoster lakes. Tarns are single lakes. Paternoster lakes are a string of lakes connected by a trunk stream that resemble beads on a rosary chain, hence, the name “Our Father” lakes.

Hanging Valley

Glacier valleys that end where they meet a deeper glacier valley and form a cliff that drops down into the more eroded valley.

Horn

High, sharp, steep-sided pyramidal peak, sculpted by cirques working headward from several sides.

Roche Moutonnée

A glacial landform that shows both abrasion and plucking. The stoss side (the side facing the glacier when it advanced over the landscape) of the bedrock hillock is abraded into a smooth bulge while the lee side is a jagged cliff due to glacial plucking.

Truncated Spur

The toe of a mountain or hill that has been eroded away by a glacier.

Glacial, or U-Shaped, Valley

These valleys are carved out by the scouring action of a glacier. The "U" shape is due to the ability of the glacier to scour on its sides as well as its base. This varies from river valleys which have a more characteristic "V" shape.

Whaleback

Smooth, glacially sculptured bedrock knob of modest size resembling the back of a whale. They often occur in “schools” and are good places to search for smaller-scale erosion features, such as glacial polish, grooves, and striations.

 

How Glaciers Transport Rock

Glaciers are like young children; they rarely put things back where they found them. They carry things off and leave them somewhere else. In short, glaciers are transporters. The “things” that glaciers transport can range in size from very small rock fragments to huge boulders, the size of cars and houses. Glaciers transport things along their bases (subglacial transport), at their surfaces (supraglacial transport), and within their ice bodies (englacial transport).

Subglacial

Particles that are not encased in a glacier’s ice may be moved as individual pieces across the bed by means of traction, slipping and rolling under propulsion from the overriding glacier.

Supraglacial

Glaciers may acquire debris as material falling on to the ice surface from rock walls or other ice-free areas. Obviously, supraglacial debris is likely to be abundant in valley and cirque glaciers and absent over large areas of ice sheets. Supraglacial debris is carried on the surfaces of glaciers. Debris can continue to be carried on a glacier’s surface within the ablation zone but above the equilibrium line, it will become progressively buried because of the accumulation of ice from above.

Englacial

Once debris becomes buried, it becomes englacial debris and can travel as such to a glacier’s snout. Alternatively, it may emerge at the surface in the ablation zone through melting of overlying ice or it can move down to the glacier bed and be trapped by existing subglacial debris.

 

How Glaciers Deposit Rock

The material that glaciers have carried can be released by a variety of processes. Material transported within glaciers (englacial debris) cannot be deposited until it reaches the base, surface, or margins of a glacier, so englacial deposition is a misnomer. Depositional processes can be grouped by whether they operate at the base (subglacial deposition), the surface (superglacial deposition), or around the margins (marginal deposition) of a glacier. Debris that originated in or on a glacier may also be carried far from the glacier’s margins (proglacial deposition). Of course, material deposited by one process may be subsequently re-entrained, re-transported, and re-deposited elsewhere by another process. In this way, erosion, transportation, and deposition all happen repeatedly and simultaneously.

Subglacial

The heat generated by friction as glaciers slip across their beds causes melting and plays a major role in deposition.

What Causes Friction Under Glaciers? First, any topographic irregularity on the bed increases the friction generated by basal sliding and thus increases melting and the deposition of debris. Second, variations in water along the bed can significantly influence deposition. Water varies widely in amount and distribution for several reasons, bed permeability being a primary factor. Deposition occurs over permeable spots more readily because the paucity of water causes increased friction, greater melting, and stronger drag on the overriding ice. Third, debris-rich ice generates more friction than clean ice.

Lodgement is another mechanism by which debris is deposited under a glacier. It entails the smearing of predominantly fine material on the bed. Lodgement of subglacial debris occurs where friction between a particle being transported by the ice and the glacier bed becomes so great that its further movement is retarded. Particles typically become lodged in some favorable spot, such as a hollow in the bed, where other particles have probably already accumulated.

Melting may also arise from geothermal heat at the base of a glacier. In volcanically active areas, such as Mt. Rainier at present and Yellowstone in the past, rapid melting of large volumes of ice and the release of enormous quantities of water creates cataclysmic floods and strange depositional features. As you can imagine, volcanic activity and glaciation are uneasy bedfellows.

Superglacial Deposition

Envision a residual winter snowbank. At first it is clean and white, then as spring approaches, it gets dirtier and dirtier. Some of the dirt settles out of the air onto the bank, but much of it represents material caught up in the snow as it was scraped into a pile.

As the snowbank melts, more and more of the dirt collects on the surface, which leads to more absorption of solar radiation and increased melting. The same thing happens to glaciers. In this case, surface melting causes the debris within a glacier to end up at its surface.

Superglacial debris suffers one of two fates. It may be moved off the surface of a glacier onto the surrounding area by gravity or running water. Or it may be let down onto the ground, in situ, as the ice melts.

Marginal Deposition

Glaciers act like conveyor belts or highway paving machines as they deposit material at their margins. The form of deposition along the margins of a glacier - and the metaphorical equipment to describe it - depends on whether a glacier is stable or advancing / retreating. Deposits dumped or spread by glaciers are much more irregular, of course, because of the hodge-podge of material they deposit and their irregular paces.

Conveyor Belt

If a glacier is stable, debris will be continuously dumped from the surface in about the same location day-after-day and year-after-year. In this way, a significant accumulation of debris can form at the margins of a glacier.

Paving Machine

If a glacier is either advancing or retreating, the conveyor belt dumps the superglacial debris in the form of a sheet, much as a highway paving machine lays down a ribbon of asphalt.

Proglacial Deposition

Glaciers are still active participants in proglacial deposition, since they furnish meltwater and debris, but they get help from streams, lakes, and even seas that are in proximity. Through these associated processes, debris can be carried and deposited many miles beyond the edge of the ice. Proglacial deposits include outwash plains, glacier milk, dropstones, and varves.

 

Features of Deposition

Glaciers are able to construct some fascinating deposits and landforms out of the jumbled mess of debris they transport.

Braided Stream

Braided streams are common in glaciated landscapes and are means for proglacial deposition, that is deposition beyond a glacier's margin. The presence of a braided stream is generally thought to indicate an inability of the stream to carry all of its load, which seems logical with respect to the huge amounts of debris produced by glaciers. The network of interlacing, intertwining channels of braided streams resemble the strands of a complex braid. Sand bars separate the branching and reuniting channels.

Drumlin

A drumlin can be thought of as a type of moraine that forms under a glacier. It is a low, streamlined mound of glacial till shaped by an overriding glacier. Its longer axis is parallel to the direction to ice movement. It usually has a blunt nose pointing in the direction from which the ice approached and a gentler slope tapering in the other direction.

Erratics

In many glaciated areas, large boulders end up stranded when glaciers recede. These out-of-place rocks are called erratics. They testify to the effectiveness of glacial deposition and lie scattered on bedrock surfaces different from their own compositions. Some erratics have glacial striations and polish, which attests to the effectiveness of glacial erosion and transport.

Esker

An esker is a long, low ridge of sand and gravel that was deposited by meltwater in a subglacial ice tunnel or an ice-walled channel. They are the classic linear feature, although they may be sinuous and/or discontinuous in reality, which generally show a close correspondence with the most recent direction of ice movement.

Kame

Kame is a broad term describing a mound of sediment formed by the initial deposition of material within a cavity in the ice followed by slumping of this material as the supporting ice walls melt away.

Outwash, Kettles, and Kettle Ponds

During a warming trend as a glacier recedes, a stream of sediment is washed out from the glacier and deposited in a flat area below, forming a deposit of outwash. In mountainous areas, these deposits are called valley trains. In flatter terrain, they are called outwash plains. Depressions, known as kettles, often pockmark outwash, and sometimes moraines. Kettles form when a block of stagnant ice becomes wholly or partially buried in sediment and ultimately melts, leaving a pit behind. Kettles can be feet or miles long but are usually shallow. In many cases, water eventually fills the depression and forms a pond or lake, called a kettle pond or lake.

Till

Till is an ill-sorted mixture of fine and coarse rock debris deposited directly from glacial ice. Terms such as “drift” and “glacial diamictite” have also been used to describe this terrestrial, glacial sediment. Chunks of till may show signs of frequent breakage that they received during transport or partial smoothing by abrasion.

Moraines

Moraines are the quintessential feature of glacial deposition. They are accumulations of till and other sediments in a dizzying array of forms. They can be parallel or perpendicular to ice movement. They can be spread across the landscape, squeezed into flutes, corrugated, hummocky, or built into high ridges.

Moraines can be extremely short-lived. They can have ice cores and collapse into insignificant forms when the core melts. They can be destroyed by meltwater, advances of subsequent glaciers, or collapse after the loss of support from glaciers that retreat. In truth, we are lucky to have them preserved in many national parks. Quick! Click on a type of moraine to learn more about it before it disappears.

End and Recessional

End moraines are transverse ridges, or groups of ridges, which delimit former ice-front positions. They are the convex part of the moraine system, along with lateral moraines, that outlines a glacier’s margin. They include recessional moraines, which represent each stabilized position of a glacier as it retreats up-valley. A valley many have 15 or 20 recessional moraines, and the size of each reflects the duration of ice stability.

Ground

Ground moraine is parallel to the direction of ice flow and is typically a formless sheet of till. It can, however, be formed into flutes on the lee-side of an embedded boulder underneath a glacier.

Lateral

Lateral moraines are formed primarily from frost-shattered debris that has fallen on the edge of a glacier from the adjacent rock walls. They are more likely to survive since they may be only partly resting upon the ice surface. Lateral moraines run parallel to the direction of ice flow and lie at the sides of a glacier. They ultimately merge with an end moraine. Lateral moraines may continue up-valley for miles.

Medial

Medial moraines have a slim chance of survival after a glacier has departed. Medial moraines are formed from debris on the surface of a glacier that is concentrated in a thin ribbon in mid-glacier below the confluence of two tributary glaciers.

Terminal

If an end moraine lies at the point of farthest glacial advance, it is the terminal moraine.

 

Links

Glaciers Index

Introduction to Glaciers

Ice Ages

Monitoring Glaciers

Glaciers and National Parks

 

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