OVERVIEW

This chapter examines the role of glacial ice as a denudation and landforming agent. Glacial ice had a major impact on the landscapes of midlatitude and subarctic regions during the past Ice Age, and still covers many high latitude and high elevation areas of the earth.

• Glaciers are natural bodies of land ice that have, or have had in the past, the ability to flow. They form where winter snowfall exceeds summer ablation over long periods of time.
• Glaciers erode the land surface by plucking and abrasion. Eroded material is incorporated into the glacier, transported and eventually deposited when the ice melts.
• Alpine glaciers form in cirques in high mountain locations and often flow down pre-existing stream valleys carving them into U-shaped glacial troughs.
• Ice sheets are accumulations of ice that cover large areas and extend over major topographic features. Greenland and Antarctica are sites of present-day ice sheets.
• Ice shelves are extensions of ice sheets that float on ocean water. Icebergs are pieces of ice that break free from ice shelves and glaciers to float in the ocean.
• Continental ice sheets expand and contract during an ice age, causing alternating periods of glaciation, deglaciation and interglaciation.
• During the Late-Cenozoic Ice Age, extending over the past 2 to 3 million years, continental ice sheets have grown and melted up to 30 times.
• Much of North America and Europe, as well as parts of Asia and South America, were covered with ice during the most recent episode of ice sheet expansion, the Wisconsin Glaciation.
• The erosive action of alpine glaciers and ice sheets produces grooved, scratched and polished surfaces on more resistant rock, and strips away regolith and weaker rock.
• Glacial drift refers to all those sediments that are deposited by glaciers. Unstratified drift deposited directly from glaciers is called till. Those sediments derived from glaciers, but modified by transportation by meltwater, are called stratified drift.
• Some of the more common landforms made up of till deposits are moraines, till plains and drumlins.
• Outwash plains form where braided meltwater streams issuing from glaciers deposit sediment over a wide area.
• Sediment deposited by meltwater streams flowing in ice tunnels beneath a glacier form ridge-like eskers.
  • Kames are stratified drift deposits that originate as deltas in meltwater lakes near glacier margins.
  • Three possible causes of the Late-Cenozoic Ice Age are:
    • a change in continent positions due to plate tectonic activity
    • an increase in the number and severity of volcanic erruptions
    • a reduction in solar energy output
• Cycles of glaciation appear to be related to cyclic changes in the earth's axial tilt and distance from the sun.
• Global warming has the potential to change both ablation and snowfall on the earth's ice sheets. The net effect of these changes on global sea level is uncertain.

KEY TERMS

ablation          moraine            Late-Cenozoic Ice  
alpine glacier    glacial trough     Age                
abrasion          fiord              Wisconsin          
plucking          glaciation         Glaciation         
ice sheet         deglaciation       glacial drift      
cirque            interglaciation    till               
                                     unstratified       
                                     drift              

STUDY QUESTIONS

1. What conditions lead to glacier formation?
2. How and why do glaciers flow?
3. What is the difference between an alpine glacier and an ice sheet?
4. Sketch a diagram to illustrate the major erosional landforms produced by an alpine glacier.
5. Describe an alpine glacier as a matter flow system. Under what conditions will the glacier front advance? Retreat?
6. Where are major ice sheets found in the world today? How large are they? How thick are they?
7. How is the Wisconsin Glaciation related to the Late-Cenozoic Ice Age?
8. How extensive were continental ice sheets during the most recent glaciation?
9. How do major glaciations affect global sea level? Why?
10. What is isostatic rebound and how has it affected the coastline of North America?
11. What is glacial drift and what are the two major types of glacial drift that are recognized?
12. Describe the major landforms formed by the deposition of till.
13. Describe the major landforms produced by the deposition of sediments from glacial meltwater.
14. What might have caused the Late-Cenozoic Ice Age?
15. What is the Holocene Epoch and how has the global climate fluctuated over this time?
16. How might global warming affect the world's major ice sheets and how might this, in turn, affect global sea levels?