17.4 Building by Ice: Glacial Deposition

Identify landforms created from glacial sediments and explain how they formed.

Glaciers and streams have many aspects in common. Both are composed of water. Both flow downslope. Both carry sediments. And both deposit sediments where they stop flowing. This section examines landforms made by glacial sediments.

Deposits by Alpine Glaciers

Alpine glaciers transport material of all sizes and pile it into jumbled, unsorted moraines composed of till. A moraine is a heap of unsorted sediments deposited by a glacier. Till is any debris deposited by a glacier without the influence of running water. In most glaciated regions, moraines are prominent landforms. There are many kinds of moraines, each identified by where it forms with respect to the movement of the glacier. Moraine types include lateral moraines, medial moraines, recessional moraines, and terminal moraines. A recessional moraine forms where the toe of the glacier pauses as it is retreating. A terminal moraine marks the farthest advance of the glacier’s toe before it begins retreating. These moraine types are illustrated in Figure 17.28.

moraine

A heap of unsorted sediments deposited by a glacier.

till

Any debris deposited by a glacier without the influence of running water.

recessional moraine

A ridge of till that forms at the toe of a glacier; formed where the glacier pauses as it is gradually retreating upslope.

terminal moraine

A moraine that marks the farthest advance of a glacier’s toe.

Figure 17.28

GEO-GRAPHIC: Moraines. (A) This diagram illustrates how different types of moraines form. (B) An outlet stream from the Piedras Blancas glacier in Patagonia, in southern Argentina, has cut a V-shaped notch into the terminal moraine at the base of the glacier. The terminal moraine has formed a natural dam behind which a tarn has formed. A lateral moraine and a recessional moraine are also visible in this photo.
(© Michael Schwab)

Moraines provide important information about the history of glaciation in a region and the history of an individual glacier. Once the age of a terminal moraine is known, estimating the average annual rate of glacial retreat is straightforward: Measure the distance between the toe of the glacier and the terminal moraine, and divide that distance by the age of the terminal moraine. Crunch the Numbers applies this method.

Deposits by Ice Sheets

We learned in Section 6.2 that Milankovitch cycles and the resulting orbital forcing cause glacial and interglacial cycles. The most recent glacial period, which is called the Wisconsin glaciation, ended about 12,000 years ago. During the Wisconsin glaciation, large ice sheets covered the high latitudes of the Northern Hemisphere (Figure 17.29). These ice sheets scoured the landscape and deposited enormous amounts of glacial drift.

Figure 17.29

Ice sheets of the Wisconsin glaciation. During the Wisconsin glaciation, the Laurentide ice sheet and the Cordilleran ice sheet covered almost all of Canada and the northernmost United States. Northern Eurasia was covered by the Scandinavian ice sheet and the Siberian ice sheet. These ice sheets were up to 3 km (2 mi) thick. Because sea level was lower during the Wisconsin glaciation, Alaska was connected to Eurasia. This map does not show the changed shape of the continents at that time.

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CRUNCH THE NUMBERS: Calculating the Rate of Glacial Retreat

CRUNCH THE NUMBERS: Calculating the Rate of Glacial Retreat

Calculate the average annual rate of retreat for the Exit Glacier in Alaska (see the opening photo in Chapter 6). It has retreated upslope 3,200 m (10,560 ft) from its terminal moraine, which has been determined by dendrochronology to be 200 years old.

  1. Question 17.10

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  2. Question 17.11

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Many areas of northern North America today are covered by nearly continuous layers of glacial sediments deposited by the Laurentide ice sheet. The sediments were deposited unevenly, resulting in an undulating, hummocky, and mounded surface called a ground moraine. In some places, these glacial deposits were molded into identifiable landforms by the moving ice sheet or by meltwater streams flowing beneath the glacier, or by a combination of ice and streams. Ice sheets produce many of the same kinds of depositional features that alpine glaciers create, such as terminal and recessional moraines. They also create landforms that are unique to ice sheets. Drumlins (Irish Gaelic for “hills”), for example, are elongated hills composed of till that was deposited by a moving ice sheet (Figure 17.30).

drumlin

An elongated hill formed by a moving ice sheet.

Figure 17.30

Drumlins. (A) Drumlins are hills shaped by a moving ice sheet that are composed mostly of clays. They are no higher than 50 m (160 ft) and a few hundred meters in length. The tapering end of the drumlin points in the direction the ice was flowing. (B) Drumlin fields, such as this one in Dane County, in southern Wisconsin, cover portions of southern Canada and the northern United States. The ice sheet movement direction was toward the viewer.
(B. © Kevin Horan/The Image Bank/Getty Images)

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Another common landform deposited by ice sheets is an esker (meaning “ridge” in Irish Gaelic), a long ridge of sorted sand and gravel deposited by a subglacial stream. Eskers may run continuously for tens of kilometers in length parallel to the direction of ice sheet movement (Figure 17.31).

esker

A long ridge of sorted sand and gravel deposited by a subglacial stream.

Figure 17.31

Eskers. (A) Subglacial streams flow in tunnels beneath an ice sheet, depositing sand and gravel in their channels. After the ice sheet melts, these glacial sediments create eskers. Eskers are up to 50 m (160 ft) in height. (B) This esker is located in Manitoba, Canada. For scale, note the mature trees, which stand about 10 m (33 ft).
(B. Grambo Photography/All Canada Photos/Getty Images)

Many areas across North America consist of rounded hills called kames, which are mounded accumulations of glaciofluvial sediments. The formation of kames is not well understood, but they are thought to be formed in part as sediments accumulate on top of depressions in a melting ice sheet. After the ice melts, the sediments are piled in mounds composed of sand and gravel.

Subglacial streams flowing beneath the Laurentide ice sheet deposited sediments on flat outwash plains where they exited the glacier. Large stagnant blocks of ice that broke from the retreating ice sheet were buried in these sediments. When the blocks eventually melted, they formed depressions called kettle holes. When kames and kettle holes form in the same area, the result is kame-and-kettle topography, a landscape dominated by irregular mounds and shallow depressions or lakes. Figure 17.32 outlines the steps in the formation of kame-and-kettle topography.

kame-and-kettle topography

A glaciofluvial landscape dominated by irregular mounds and shallow depressions or lakes.

Figure 17.32

GEO-GRAPHIC: Kame-and-kettle topography formation. (A) Kame-and-kettle topography forms as an ice sheet melts. (B) Kame-and-kettle topography is found throughout much of Canada. This photo is from the Northwest Territories.
(B. © Thomas & Pat Leeson/Science Source)

In addition to creating kame-and-kettle topography, the retreat of the Laurentide ice sheet left a series of recessional moraines across much of the upper Midwest of the United States and the province of Ontario in Canada. From the ground, these recessional moraines are less easy to see than eskers, drumlins, and kettle holes. Figure 17.33 maps their extent.

Figure 17.33

North American recessional moraines. There are many recessional moraines in the Great Lakes region. The moraines mapped in the dark blue area were formed by the Laurentide ice sheet some 15,000 years ago. Moraines in southern Illinois, Indiana, and Ohio (mapped in gray) are from the previous glacial period, called the Illinoian glaciation, that occurred some 150,000 years ago. Southwestern Wisconsin does not have drift (or glaciofluvial deposits) and is called the “driftless area.” Many scientists think the glaciofluvial deposits in the driftless area were washed away long ago in a large flood event caused by the sudden draining of one of the ancient lakes in the area.

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Glacial Dust: Loess

About 10% of the surface of the continents, mostly at midlatitudes, is covered by loess deposits. Loess is made up of wind-deposited dust that often originates as glacial flour deposited on glacial outwash plains. Electrical charges on these tiny silt and clay particles cause the fine dust to stick together where it settles, forming large accumulations of loess. Deposits of loess are typically unstratified (lacking horizontal layers).

loess

(pronounced lehss) Wind-deposited silt and clay sediments that originate mostly from glacial outwash plains.

Many loess deposits were formed by processes that are no longer active. The Pleistocene ice sheets and mountain glaciers in North America and Scandinavia formed loess accumulations when summer meltwater deposited silt and clay onto the outwash plain. Cold winters reduced the stream flow, exposing the sediments to strong katabatic winds (see Section 4.4). These winds picked up and transported the fine sediments and deposited them in loess accumulations.

About 30% of the United States is covered by loess deposits, and Europe has extensive loess deposits, as shown in Figure 17.34. Not all loess is glacially derived. In some areas, such as central China, loess was formed by windblown dust originating in deserts rather than glacial outwash plains.

Figure 17.34

Global loess deposits. Loess deposits are found mostly at midlatitudes. The inset photo shows a roadcut through the Loess Hills in the Missouri River valley in western Iowa. The holes have been excavated for nesting by bank swallows.
(© Lee Rentz/NHPA/Photoshot)