Glaciers have the ability to completely transform a landscape. During the last glacial period (14, 000 years ago) most of Canada was covered in ice more than a kilometre thick! As the ice advanced over the land it plowed through the existing rock and soil and created most of the landforms that we see today. Not only can glaciers and ice sheets erode the surface of the earth but they can also deposit sediments.

Glacial Erosion:

This is a close up image of striations. Striations are erosional features. The ice at the bottom of a glacier (basal ice) is usually loaded with debris, including large rocks. As this basal ice moves over top of bedrock it scrapes the bedrock and leaves striations, or grooves behind. Geologists use these markings to infer the direction of ice motion. In this picture, the movement of ice could be either from the left to the right or from the right to the left. On close inspection of these grooves, you can sometimes see that one end of the striation is more shallow than the other, so that in cross section the striation would be deeper at one end than the other. What does this shape imply about ice movement? Do you think it moves from the left to the right or from the right to the left? (Remember that rocks embedded in the ice are what scrape the bedrock)










Deposition:

When a glacier moves over the land it erodes a great deal of material. The deposits that the glacier leaves behind are a mixture of all of the different materials that it eroded. This material is called till and is composed of clay, sand and gravel. You can tell a lot about the path of a glacier by examining the composition of the till beneath it. If a till mixture is composed of a special type of rock only found in one region then the glacier must have moved over that region. If you had to guess...how much of North America do you think is covered in till deposits? What economical use does this material have?

Busting Rocks Experiment

Materials: rocks several pieces of cloth hammer plastic milk carton plaster of Paris large matchbox pencil freezer

When water freezes it occupies a larger volume. As freezing water expands, it exerts a force. Such a force may widen cracks in rocks and eventually break the rocks apart. Such changes are physical changes, just as freezing water is a physical change. Freezing water splits rocks from mountains. These rocks may be split again and again, and end up as small pebbles carried away by streams. The action of freezing water is a type of physical weathering, one of several ways rocks are broken down.

Procedure:

1. Wrap several rocks in several pieces of cloth. Place them on a sidewalk and have the students hit them several times each with the hammer. Unwrap and examine the particles; compare them with sand.

What is sand?

2. Fill an empty milk carton to the top with water. Close the top securely and place the carton in a freezer until the next day.

What do you observe?

What caused it?

How might this lead to erosion on a mountain?

3. Fill a large matchbox with plaster of Paris paste. With a pencil, make a one-inch deep groove in the block before it dries. The groove should not extend to the ends of the block. Allow the block to dry. Measure and record the width and length of the groove. Fill the groove with water and place the block in the Freezer. After the water has frozen, remeasure the groove and record.

Are there any changes in the size of the groove?

4. Allow the ice to melt and refreeze.

How many times do you have to repeat the freezing until the block cracks?

How does freezing water break rocks from mountains?

5. Soak a soft red brick or a piece of sandstone in water overnight. Place it in a pan and put it in a freezer overnight.

What changes are observed?

6. Repeat the thawing and freezing several times and observe.

This lesson came from: Sund, R., Tillery, B., & Trowbridge, L. (1973) Elementary Science Discovery Lessons: The Earth Sciences. Boston, MA: Allyn and Bacon, Inc.

Glacial Landforms:

This picture shows the area in front of the Athabasca glacier in the Rockies. You can actually drive right up to this glacier on the Banff/Jasper highway. The glacier used to be a lot bigger than it is now. You can't even see it in the photo now. The dates on the photo associated with the black lines indicate the position of the glacier at that time. The ridges in the upper right of the image are called moraines. They are used by geologists to indicate the position of the glacier at different times in glacial history. Do you think that the highway was there in 1870? Why/why not? Why is there a big lake in front of the glacier? Where did all of this water come from? Glaciologists use the past positions of glaciers to estimate what the climate used to be in the area. If the glacier was much bigger 1000 years ago, then this implies that temperatures were also much cooler 1000 years ago. When temperatures increase to a point where more ice melts than is formed in the wintertime then the glacier "retreats". Although the word retreat implies that the glacier is moving backwards, it isn't. A glacier doesn't move backwards, it just melts...giving the appearance that it is moving backwards. Almost all of the glaciers in North America (and probably the world) are currently in retreat. What does this imply about global temperature in the last couple hundred years?