10.4 Erosion and Deposition by Glaciers

Lesson Objectives

• Describe how continental and valley glaciers form.
• Explain how glaciers cause erosion.
• Identify landforms deposited by glaciers.

Vocabulary

• continental glacier
• glacial till
• glacier
• moraine
• plucking
• valley glacier

Introduction

Glaciers are masses of flowing ice. Today, they cover only about 10 percent of Earth’s surface. They are getting smaller and smaller as Earth’s temperature rises. But just 12,000 years ago, glaciers dipped as far south as Chicago and New York City. Much of Europe was also covered with glaciers at that time.

Glaciers erode and leave behind telltale landforms. These landforms are like clues. They show the direction a glacier flowed and how far it advanced. Did glaciers leave clues where you live? Would you know what to look for?

How Glaciers Form

Glaciers form when more snow falls than melts each year. Over many years, layer upon layer of snow compacts and turns to ice. There are two different types of glaciers: continental glaciers and valley glaciers. Each type forms some unique features through erosion and deposition. An example of each type is pictured in Figure below.

• A continental glacier is spread out over a huge area. It may cover most of a continent. Today, continental glaciers cover most of Greenland and Antarctica. In the past, they were much more extensive.
• A valley glacier is long and narrow. Valley glaciers form in mountains and flow downhill through mountain river valleys.

(A) The continent of Antarctica is covered with a continental glacier. (B) A valley glacier in the Canadian Rockies. (C) The surface of a valley glacier.

Erosion by Glaciers

Like flowing water, flowing ice erodes the land and deposits the material elsewhere. Glaciers cause erosion in two main ways: plucking and abrasion.

• Plucking is the process by which rocks and other sediments are picked up by a glacier. They freeze to the bottom of the glacier and are carried away by the flowing ice.
• Abrasion is the process in which a glacier scrapes underlying rock. The sediments and rocks frozen in the ice at the bottom and sides of a glacier act like sandpaper. They wear away rock. They may also leave scratches and grooves that show the direction the glacier moved.

Erosion by Valley Glaciers

Valley glaciers form several unique features through erosion. You can see some of them in Figure below.

• As a valley glacier flows through a V-shaped river valley, it scrapes away the sides of the valley. It carves a U-shaped valley with nearly vertical walls. A line called the trimline shows the highest level the glacier reached.
• A cirque is a rounded hollow carved in the side of a mountain by a glacier. The highest cliff of a cirque is called the headwall.
• An arête is a jagged ridge that remains when cirques form on opposite sides of a mountain. A low spot in an arête is called a col.
• A horn is a sharp peak that is left behind when glacial cirques are on at least three sides of a mountain.

Features Eroded by Valley Glaciers. Erosion by valley glaciers forms the unique features shown here.

Deposition by Glaciers

Glaciers deposit their sediment when they melt. They drop and leave behind whatever was once frozen in their ice. It’s usually a mixture of particles and rocks of all sizes, called glacial till. Water from the melting ice may form lakes or other water features. Figure below shows some of the landforms glaciers deposit when they melt.

• Moraine is sediment deposited by a glacier. A ground moraine is a thick layer of sediments left behind by a retreating glacier. An end moraine is a low ridge of sediments deposited at the end of the glacier. It marks the greatest distance the glacier advanced.
• A drumlin is a long, low hill of sediments deposited by a glacier. Drumlins often occur in groups called drumlin fields. The narrow end of each drumlin points in the direction the glacier was moving when it dropped the sediments.
• An esker is a winding ridge of sand deposited by a stream of meltwater. Such streams flow underneath a retreating glacier.
• A kettle lake occurs where a chunk of ice was left behind in the sediments of a retreating glacier. When the ice melted, it left a depression. The meltwater filled it to form a lake.

Take a look at the glacial deposits. How far did the glacier in the diagram advance before it started retreating?

Lesson Summary

• Glaciers are masses of flowing ice. Continental glaciers are huge. They may spread out over much of a continent. Valley glaciers are long and narrow. They form in mountains and flow through mountain river valleys.
• Glaciers cause erosion by plucking and abrasion. Valley glaciers form several unique features through erosion, including cirques, arêtes, and horns.
• Glaciers deposit their sediment when they melt. Landforms deposited by glaciers include drumlins, kettle lakes, and eskers.

Lesson Review Questions

Recall

1. What is a glacier?

2. Describe how glaciers form.

3. Identify the two main ways glaciers cause erosion.

4. Name and describe three unique features eroded by valley glaciers.

5. What is glacial till?

Apply Concepts

6. Create a lesson to teach younger students how a kettle lake forms. Outline your lesson.

Think Critically

7. Compare and contrast valley and continental glaciers and how they change Earth’s surface.

8. Areas once covered by glaciers may have large boulders called erratics, like the one in the photo below. Infer why erratics typically consist of a different type of rock than the bedrock where they are found.

Points to Consider

So far in this chapter, you’ve read how moving water, air, and ice shape Earth’s surface. Water and ice move because of gravity.

• Do you think gravity can erode and deposit sediment without the help of water or ice?
• How might gravity alone shape Earth’s surface?