Learning Objectives

Each statement represents the primary learning objective for the corresponding major heading within the chapter. After you complete the chapter, you should be able to:

7.1 Explain the role of glaciers in the hydrologic and rock cycles and describe the different types of glaciers and their present-day distribution.

7.2 Describe how glaciers move, the rates at which they move, and the significance of the glacial budget.

7.3 Discuss the processes of glacial erosion and the major features created by these processes.

7.4 Distinguish between the two basic types of glacial deposits and briefly describe the features associated with each type.

7.5 Describe and explain several important effects of Ice Age glaciers other than the formation of erosional and depositional landforms.

7.6 Discuss the extent of glaciation and climate variability during the Quaternary Ice Age.

7.7 Describe the general distribution and causes of Earth’s dry lands and the role that water plays in modifying desert landscapes.

7.8 Contrast the landscapes of the Basin and Range and the Colorado Plateau in the western United States.

7.9 Describe the ways in which wind transports sediment and the features created by wind erosion. Distinguish between two basic types of wind deposits.

Concept Checks

7.1 Where are glaciers found on Earth today, and what percentage of Earth’s land surface do they cover? Describe how glaciers fit into the hydrologic cycle. What role do they play in the rock cycle? List and briefly distinguish among four types of glaciers. What is the difference between an ice sheet, sea ice, and an ice shelf?

7.2 Describe two components of glacial movement. How rapidly does glacial ice move? Provide some examples. What are crevasses, and where do they form? Under what circumstances will the front of a glacier advance? Retreat? Remain stationary? 

7.3 How do glaciers acquire their load of sediment? How does a glaciated mountain valley differ in appearance from a mountain valley that was not glaciated? Describe the features created by glacial erosion that you might see in an area where valley glaciers recently existed. 

7.4 What is the difference between till and stratified drift? Describe the formation of a medial moraine. Distinguish among terminal end moraine, recessional end moraine, and ground moraine. Relate these moraines to the budget of a glacier. List four depositional features other than moraines. 

7.5 List four effects of Ice Age glaciers, aside from the formation of major erosional and depositional features. Examine Figure 7.19 and determine how much sea level has changed since the Last Glacial Maximum. Contrast proglacial lakes and pluvial lakes. 

7.6 About what percentage of Earth’s land surface was affected by glaciers during the Quaternary period? Where were ice sheets more extensive during the Ice Age: the Northern Hemisphere or the Southern Hemisphere? Why? 

7.7 Define dry climate. How extensive are the desert and steppe regions of Earth? Discuss the causes of deserts. What is an ephemeral stream? What is the most important erosional agent in deserts?

7.8 Describe the features and characteristics associated with each stage in the evolution of a mountainous desert. Where in the United States can each stage of desert landscape evolution be observed? How does the landscape of the Colorado Plateau differ from that of the Basin and Range? 

7.9 Why is wind erosion relatively more effective in arid regions than in humid areas? What are blowouts? What term describes the process that creates these features? Contrast loess and sand dunes in terms of composition and how they form. Describe how sand dunes migrate. 

Concepts In Review Glacial and Arid Landscapes

7.1 Glaciers and the Earth System

Explain the role of glaciers in the hydrologic and rock cycles and describe the different types of glaciers and their present-day distribution.

Key Terms: glacier, valley (alpine) glacier, ice sheet, sea ice, ice shelf, ice cap, piedmont glacier, outlet glacier

*A glacier is a thick mass of ice originating on land from the compaction and recrystallization of snow, and it shows evidence of past or present flow. Glaciers are part of both the hydrologic cycle and the rock cycle. They store and release freshwater, and they transport and deposit large quantities of sediment.

*Valley glaciers flow down mountain valleys, whereas ice sheets are very large masses, such as those that cover Greenland and Antarctica. During the Last Glacial Maximum, about 18,000 years ago, large areas of Earth were covered by glacial ice.

*When valley glaciers exit confining mountains, they may spread out into broad lobes called piedmont glaciers. Similarly, ice shelves form when glaciers flow into the ocean, producing a layer of floating ice.

*Ice caps are like small ice sheets. Both ice sheets and ice caps may be drained by outlet glaciers, which often resemble valley glaciers.

QUESTION: This satellite image shows ice in the high latitudes of the Northern Hemisphere. What term is applied to the ice at the North Pole? What term best describes Greenland’s ice? Are both considered glaciers? Explain. (NASA) 

7.2 How Glaciers Move

Describe how glaciers move, the rates at which they move, and the significance of the glacial budget.

Key Terms: crevasse, zone of accumulation, zone of wastage, calving, iceberg, glacial budget

*When ice is put under pressure, it flows very slowly. In the uppermost 50 meters (165 feet) of a glacier, there is not sufficient pressure to flow, and cracks called crevasses open up in the zone of fracture. In addition, most glaciers also move by sliding across the ground.

*Fast glaciers may move 800 meters (2600 feet) per year, while slow glaciers may move only 2 meters (6.5 feet) per year. Some glaciers experience periodic surges of rapid movement.

*When a glacier has a positive budget, the terminus advances. This occurs when the glacier gains more snow in its zone of accumulation that it loses at its zone of wastage. If wastage exceeds the input of new ice, the glacier’s terminus retreats.

QUESTION: The image shows that melting is one mechanism by which glacial ice wastes away. What is another way that ice is lost from a glacier? (Robbie Shone/Science Source) 

7.3 Glacial Erosion

Discuss the processes of glacial erosion and the major features created by these processes.

Key Terms: plucking, abrasion, rock flour, glacial striations, glacial trough, hanging valley, cirque, arête, horn, fiord

*Glaciers acquire sediment through plucking from the bedrock beneath the glacier, by abrasion of the bedrock using sediment already in the ice, and when mass-movement processes drop debris on top of the glacier. Grinding of the bedrock produces grooves and scratches called glacial striations.

*Erosional features produced by valley glaciers include glacial troughs, hanging valleys, cirques, arêtes, horns, and fiords.

QUESTION: Examine the illustration of a mountainous landscape after glaciation. Identify the landforms that resulted from glacial erosion. Streams occupy the locations where glaciers once existed. Small lakes occupy small circular basins at higher elevations. Sharp ridges separate the small round basins from one another. The main valley is U shaped. Tributary valleys abruptly slope down into the main valley. At the highest elevations, there are sharp peaks.

7.4 Glacial Deposits

Distinguish between the two basic types of glacial deposits and briefly describe the features associated with each type.

Key Terms: glacial drift, till, glacial erratic, stratified drift, lateral moraine, medial moraine, end moraine, ground moraine, outwash plain, valley train, kettle, drumlin, esker, kame

*Any sediment of glacial origin is called drift. The two distinct types of glacial drift are (a) till, which is unsorted material deposited directly by the ice, and (b) stratified drift, which is sediment sorted and deposited by meltwater from a glacier.

*The most widespread features created by glacial deposition are layers or ridges of till, called moraines. Associated with valley glaciers are lateral moraines, formed along the sides of the valley, and medial moraines, formed between two valley glaciers that have merged. End moraines, which mark the former position of the front of a glacier, and ground moraines, undulating layers of till deposited as the ice front retreats, are common to both valley glaciers and ice sheets.

QUESTION: Examine the illustration of depositional features left behind by a retreating ice sheet. Identify the features and indicate which landforms are composed of till and which are composed of stratified drift. A till layer underlies the foreland of a retreating glacier. A small, narrow elevated landform snakes out from the front of the retreating glacier. Round elevated bumps jut out in the foreground, along with rounded, elongated features. A river with various channels flows out of the terminus of the retreating glacier. Two ridges rise along the length of the retreating glacier in its wake. Round lakes are between the ridges. A plain is on the other side of the ridges from the retreating glacier. 

7.5 Other Effects of Ice Age Glaciers

Describe and explain several important effects of Ice Age glaciers other than the formation of erosional and depositional landforms.

Key Terms: proglacial lake, pluvial lake

*Ice sheets are nourished by water that ultimately comes from the ocean, so when ice sheets grow, sea level falls, and when they melt, sea level rises.

*Advance and retreat of ice sheets caused significant changes to the paths followed by rivers. Proglacial lakes formed when glaciers acted as dams to create lakes by trapping glacial meltwater or blocking rivers. In response to the cooler and wetter glacial climate, pluvial lakes formed in areas such as present-day Nevada.

7.7 Deserts

Describe the general distribution and causes of Earth’s dry lands and the role that water plays in modifying desert landscapes.

Key Terms: dry climate, desert, steppe, ephemeral stream

*Dry climates cover about 30 percent of Earth’s land area. These regions have yearly precipitation totals that are less than the potential loss of water through evaporation. Deserts are drier than steppes, but both climate types are considered water deficient.

*Dry regions in the lower latitudes coincide with zones of subsiding air and high air pressure known as subtropical highs. Middle-latitude deserts exist because of their positions in the deep interiors of large continents far removed from oceans. Mountains also act to shield these regions from humid marine air masses.

*Practically all desert streams are dry most of the time and are said to be ephemeral. Nevertheless, running water is responsible for most of the erosional work in a desert. Although wind erosion is more significant in dry areas than elsewhere, the main role of wind in a desert is transportation and deposition of sediment.

7.8 Arid Landscapes of the American West

Contrast the landscapes of the Basin and Range and the Colorado Plateau in the western United States.

Key Terms: interior drainage, alluvial fan, bajada, playa lake, mesa, butte, pinnacle

*The Basin and Range region of the western United States is characterized by interior drainage with streams eroding uplifted mountain blocks and depositing sediment in interior basins. Alluvial fans, bajadas, playas, playa lakes, salt flats, and inselbergs are features often associated with these landscapes.

*The Colorado Plateau is an elevated region of relatively flat-lying strata that has hundreds of deep canyons. When plateau cliffs composed of resistant sandstone and limestone retreat, mesas, buttes, and pinnacles gradually form.

QUESTION: Identify the lettered features in this photo. How did they form? (Michael Collier) 

7.9 The Work of Wind

Describe the ways in which wind transports sediment and the features created by wind erosion. Distinguish between two basic types of wind deposits.

Key Terms: deflation, blowout, loess, dunes, lip, face, cross bedding

*For wind erosion to be effective, dryness and scant vegetation are essential. Deflation, the lifting and removal of loose material, often produces shallow depressions called blowouts and can also lower the surface by removing sand and silt.

*Abrasion, the “sandblasting” effect of wind, is often given too much credit for producing desert features. However, abrasion does cut and polish rock near the surface.

*Wind deposits are of two distinct types: (1) extensive blankets of silt, called loess, that form when wind deposits its suspended load, and (2) mounds and ridges of sand, called dunes, that form when sediment that was carried as part of the wind’s bed load accumulates.