Glacial Landscapes 5:Glaciers, Deserts, and Wind

Cirques

A cirque is a bowl-shaped depression at the head of a glacial valley that is surrounded on three sides by steep rock walls. These impressive features are the focal point of the glacier's growth because they form where snow and ice accumulate at the head of a valley glacier. Cirques begin as irregularities in the mountainside. Glaciers carve cirques by plucking rock from along the sides and the bottom. The glaciers then act as conveyor belts that carry away the debris. Sometimes the melting glacier leaves a small lake in the cirque basin.

What kind of surface feature do glaciers form as they recede?

As a glacier recedes, it forms a ground moraine—a rolling plain.

Variables of glacial erosion

As with other agents of erosion, the rate of glacial erosion is highly variable. These differences are mainly controlled by four factors: 1) rate of glacial movement; 2) thickness of the ice; 3) shape, abundance, and hardness of the rock fragments in the ice at the base of the glacier; and 4) the type of surface below the glacier.

Outwash Plains

At the same time that an end moraine is forming, streams of fast-moving meltwater emerge from the bases of glaciers. This water is often so choked with fine sediment that it looks like milk. After it leaves the glacier, the water slows and drops the sediment in a broad, ramp-like accumulation downstream from the end moraine. This type of sediment ramp resulting from an ice sheet is called an outwash plain.

Glaciated Valleys 1

Before glaciation, alpine valleys are usually V-shaped because streams are well above base level and are downcutting. However, in mountain regions that have been glaciated, the valleys are no longer narrow. As a glacier moves down a valley once occupied by a stream, the glacier widens, deepens, and straightens the valley. The once narrow V-shaped valley is changed into a U-shaped glacial trough.

Till Boulders

Boulders found in till or lying free on the ground are glacial erratics. Their mineral content is different from the underlying bedrock, which shows they were carried there by some means. In parts of New England and other glaciated areas, glacial erratics are scattered throughout pastures and farm fields. Early settlers cleared the smaller ones from their fields and piled them into stone fences that remain today. Geologists can sometimes determine the path of a long-gone glacier by studying the minerals in glacial erratics.

Drumlins and Eskers 2

Drumlins are streamlined hills composed of till. Drumlins are taller and steeper on one end, and they range in height from 15 to 60 meters and average 0.4 to 0.8 kilometer long. The steep side of the hill faces the direction the ice came from, and the gentler slope points in the direction the ice moved. Drumlins occur in clusters called drumlin fields. Near Rochester, New York, one cluster contains nearly 10,000 drumlins. The streamlining shows they were molded by active glaciers.

End Moraines and Ground Moraines 4

End moraines that formed in the recent ice age are prominent in the landscapes of the Midwest and Northeast. The Kettle Moraine is a scenic one that occurs in Wisconsin near Milwaukee. New York's Long Island is part of a series of end moraines stretching from eastern Pennsylvania to Cape Cod, Massachusetts. The figure shows the locations of these end moraines that form part of the Northeast coast.

Landforms Created by Glacial Erosion

Erosion by valley glaciers produces many spectacular features in mountainous areas. Glaciers are responsible for a variety of erosional landscape features, such as glacial troughs, hanging valleys, cirques, arêtes, and horns.

Eskers

Eskers are snake-like ridges composed of sand and gravel that were deposited by streams once flowing in tunnels beneath glaciers. They can be several meters high and many kilometers long. Many eskers are mined for the sand and gravel they contain.

Till

Glacial drift applies to all sediments of glacial origin, no matter how, where, or in what form they were deposited. There are two types of glacial drift: till and stratified drift. Till is material deposited directly by the glacier. It is deposited as the glacier melts and drops its load of rock debris. Unlike moving water and wind, ice cannot sort the sediment it carries. Therefore, till deposits are usually unsorted mixtures made up of many particle sizes.

What is glacial drift?

Glacial drift is sediment that is deposited by a glacier. There are two types of glacial drift: till and stratified drift.

Glacial Erosion

Glaciers are nature's bulldozers. Their ice scrapes, scours, and tears rock from valley floors and walls. Glaciers then carry the rocks down the valley. The rock fragments that are eroded by the glacier drop at the glacier's foot where the ice melts. Unlike streams, which drop sediments while they flow, glaciers hold everything until they melt. They can carry rocks as big as buses over long distances. Many landscapes were changed by the widespread glaciers of the recent ice age.

Moraines, Outwash Plains, and Kettles

Glaciers are responsible for a variety of depositional features, including moraines, outwash plains, kettles, drumlins, and eskers. When glaciers melt, they leave layers or ridges of till called moraines. These widespread glacial features come in several varieties.

Terminal and Recessional Moraines

Glaciers can periodically retreat, then find equilibrium again and remain stationary for some time. A glacier forms a new end moraine during the stationary period, then another ground moraine after it starts retreating again. This pattern can repeat many times before the glacier completely melts. The farthest end moraine is the terminal end moraine. The end moraines that form when the ice front occasionally becomes stationary during its retreat are recessional end moraines.

End Moraines and Ground Moraines 1

Glaciers can remain stationary for long periods of time. When a glacier is stationary it means snow and ice accumulate at the head of the glacier at the same rate snow and ice melt at the foot of the glacier.

How Glaciers Erode 1

Glaciers mainly erode the land in two ways: plucking and abrasion. Rock surfaces beneath glaciers break up as melted water from the glacier penetrates the cracks. When the water refreezes, it expands and pries the rock apart. As a glacier flows over the fractured bedrock surface, it loosens and lifts blocks of rock and incorporates them into the ice. This type of glacial erosion is called plucking.

Glacial Deposits

Glaciers transport huge loads of debris as they slowly advance across the land. When a glacier melts it deposits its sediment. For example, in many areas once covered by the ice sheets of the recent ice age, the bedrock is rarely exposed because glacial deposits that are dozens—or even hundreds—of meters thick completely cover the terrain. Rocky pastures in New England, wheat fields in the Dakota plains, and rolling Midwest farmland are all landscapes resulting from glacial deposition.

End Moraines and Ground Moraines 3

Ground moraines form when glaciers begin to recede. The glacier front continues to deliver debris. The glacier deposits sediment as the ice melts away. However, instead of forming a ridge, the retreating glacier creates a rock-strewn, gently rolling plain. This ground moraine fills in low spots and clogs old stream channels. Ground moraine can thus result in poorly drained swamplands.

How are kettles formed?

Kettles are formed when a piece of glacier ice breaks off and becomes buried by till or moraine deposits. Over time the ice melts, leaving a small depression in the land, filled with water.

Lateral Moraines 2

Medial moraines are formed when two valley glaciers join to form a single ice stream. The till that was once carried along the edges of each glacier joins to form a dark stripe of debris within the newly enlarged glacier.

Drumlins and Eskers 1

Moraines are not the only landforms deposited by glaciers. Some landscapes have many elongated parallel hills made of till. Other areas have conical hills and narrow winding ridges made mainly of stratified drift. If you know what to look for, the signs of a once-glaciated landscape are unmistakable—especially from an airplane.

Arêtes and Horns

Other mountain landscapes carved by valley glaciers reveal more than glacial troughs and cirques. Snaking, sharp-edged ridges called arêtes and sharp pyramid-like peaks called horns project above the surroundings. You can see these features in the Alps and the northern Rockies. Horns like the Matterhorn in Switzerland form where several cirques surround a single high mountain. The converging cirques create one distinctive horn. Arêtes form where cirques occur on opposite sides of a divide. As these cirques grow, the divide separating them is reduced to a narrow, sharp ridge.

How does glacial erosion through plucking take place?

Plucking takes place when melted water from the glacier penetrates the cracks in rocks, the rock surfaces break up. On refreezing, this water expands and pries the rock apart. As a glacier flows over this fractured rock surface, it lifts the loose rocks and incorporates them into ice.

Stratified

Stratified drift is sediment laid down by glacial meltwater. Stratified drift contains particles that are sorted according to the size and weight of the debris. Some deposits of drift are made by streams coming directly from the glacier. Stratified drift often consists of sand and gravel, because the meltwater cannot move large boulders. Finer sediments remain suspended and are carried far from the glacier.

Glaciated Valleys 2

The amount of glacial erosion depends in part on the thickness of the ice. Main glaciers cut U-shaped valleys that are deeper than those carved by smaller side glaciers. When the ice recedes, the valleys of the smaller side glaciers are left standing higher than the main glacial trough. These higher valleys are called hanging valleys. Rivers flowing from hanging valleys sometimes produce spectacular waterfalls, such as those in Yosemite National Park, California.

How Glaciers Erode 2

The second form of glacial erosion is called abrasion. As the glacial ice and its load of rock fragments slide over bedrock, they work like sandpaper to smooth and polish the surface below. The pulverized rock produced by this glacial gristmill is appropriately called rock flour. So much rock flour may be produced that streams of meltwater leaving the glacier often have the grayish appearance of skim milk—visible evidence of the grinding power of the ice.

Lateral Moraines 1

The sides of a valley glacier gather large amounts of debris from the valley walls.Lateral moraines are ridges that form along the sides of glacial valleys when the glacier melts and leaves the material it has gathered.

How Glaciers Erode 3

When the ice at the bottom of a glacier contains large rock fragments, long scratches and grooves may be gouged in the bedrock. These glacial striations provide valuable clues to the direction of past glacial movement. By mapping the striations over large areas, geologists often can reconstruct the direction the ice flowed.

End Moraines and Ground Moraines 2

Within the glacier, the ice still flows. It acts as a conveyor belt to carry rock debris to the end of the glacier. When the ice there melts, it deposits the debris and forms a ridge called an end moraine. The longer the glacier remains stationary, the larger the end moraine grows.

Kettles

You can often find depressions and small lakes called kettles within end moraines and outwash plains. Kettles form when blocks of stagnant ice become buried in drift and eventually melt. The melting leaves pits in the glacial sediment. A well-known example of a kettle is Walden Pond near Concord, Massachusetts. Thousands of kettles dot the landscape of the Upper Midwest in Wisconsin and Minnesota.

Minnesota and Wisconsin have thousands of kettles. How does a kettle form?

a block of glacial ice is buried, then melts

Glacial erosion can shape many landforms, including cirques. What is a cirque?

a bowl-shaped depression

Till is deposited directly by a melting glacier. A layer or ridge of till is called _____.

a moraine

A valley glacier advances down a mountain valley and then retreats. How did the glacier most likely change the valley?

by carving a V-shape into a U-shape

During the recent ice age, glaciers covered much of the Midwest region of the United States. How did the glaciers affect lands in the Midwest that are now used for farming and ranching?

by depositing thick layers of rocks and sediment

Moraines, outwash plains, kettles, and drumlins all form from _____.

deposition by melting glaciers

Glaciers erode the land in two main ways, which are called _____.

plucking and abrasion

Which land feature do glaciers typically cause by abrasion?

smooth, polished bedrock

A scientist wants to classify a sample of glacial drift as either till or stratified drift. Which observation or measurement would be most useful for classifying the glacial drift?

the range of sizes of the sediments