Geology Activity:
How Glaciers Shape the Land

(Ardith Hansel, Illinois Geological Survey)

Objectives: To show how the abrasive action of a glacier can change the surfaces over which it flows and to show how the melting of a glacier can result in deposition of sediment.

Material: Each group of 5 will need:
>Several small, sharp stones (pebble-size road gravel)
>A handful of coarse sand
>A large plastic margarine tub or topping container (1 qt.)
>An unglazed brick or flat paving brick
>A flat limestone rock
>Water and access to a freezer
>Protective gloves

Preparation: fill each tub with 2.5 cm of water and place the sand and pebbles in the bottom. Allow the sand to settle and freeze solid. Later add more water until the tubs are almost full and again freeze solid. These will be the glaciers, which will have a lower zone of dirty ice and an upper zone of relatively clean ice.

Procedure:
1). Divide participants in groups of 5. After you have discussed glaciers with the class, state that this activity will illustrate some of the processes associated with glaciation. It is best to do this activity outside.

2). Provide each group with a glacier. These can be removed from the bowls by letting them sit in warm water or just letting them sit out for a short time. One person puts on the gloves and places the glacier with the dirty-ice-side down and pressing hard, slides it across the rock. It is necessary to do this in the same direction a number of times. Participants can take turns doing this.

3). Describe that by pressing down hard, they are essentially doing what a glacier does when it flows or slides cross the land pushing outward from its center toward its edges. Have the participants decide how the dirty ice moves across the brick. Does it scratch the brick's surface or remove any material from the surface of the brick? How can one tell? What happens to the surface of the limestone rock? What would eventually happen to the rocks at the base of the glacier if this action was repeated for a long time?

4). As a final activity, place the glaciers in a corner of the schoolyard that will not be disturbed. Let them melt. Return with the class and observe what has happened to the sediment the ice was carrying.

Background: This activity is a hands-on demonstration of the abrasive power of glaciers carrying rocks and sand and the ability of glaciers to modify the landscape. Its purpose is to illustrate the concept that glaciers change the Earths surface by the processes of erosion and deposition.

Glaciers form during climatic episodes when more snow accumulates during the winter than melts away during the summer; over time the snow thickens and under the pressure of its own weight is compressed into ice and begins to flow outward. During warm intervals, glacier ice melts and the glaciers recede (melt back). During cold intervals, more snow accumulates than melts and the glaciers advance. Rocks and soil frozen in the base of the glacier are dragged along. These materials can act like sandpaper to smooth the landscape over which the glacier passes. This abrasion can smooth the face of rocks and round out hills. It can also gouge out valleys. On some exposed rock surfaces you can find evidence of this glacial scouring in the form of glacial striations (parallel lines carved into the rock).

Like rivers, glaciers tend to follow the path of least resistance. The glaciers that flowed into the northern United States from Canada during the Ice Age followed the courses of former river valleys that were eroded into the least resistant rocks. The glaciers gouged out rock from the base and sides of these valleys creating the troughs occupied today by the Great Lakes.

When glaciers melt, they leave behind whatever they were carrying. This may consist of a mixture of rock debris and old soils, called till, material that was ground-up and deposited from the base of the glacier. Some of the most striking reminders that the landscape was once covered by glaciers are boulders and cobbles, called erratics, that dot the landscape. Unlike the rock you find in local quarries, these are exotic rocks. In Illinois, erratics are often granitic rocks like those you would find today hundreds of miles away in Canada.

Glacial meltwaters carried ground-up rock debris away from the glaciers. In the valleys of major meltwater channels, like the Mississippi and Illinois valleys, this debris settled out as layers of silt, sand, and gravel, called outwash. On dry, windy days, the finest particles of this outwash were blown across the landscape in glacial dust storms. These particles settled across the landscape to form a blanket of silt-size particles, called loess. Loess forms the basis of the young, rich soils of the northern plains of the United States.

Answers: The dirty ice scrapes off part of the surface of the brick. Brick-colored clay particles will get trapped among the sand and rocks in the base of the ice, demonstrating the erosive ability of the dirty ice to remove material from the bricks surface. The sand grains and rocks in the base of the ice will scratch the surface of the limestone, forming the fine striations. If the rock has an uneven surface, the striations will be most obvious on the highest parts of the rocks surface. Scraping of rocks embedded in the ice across a hard surface will flatten the exposed sides of protruding rocks. These flat faces on pebbles and cobbles are called facets. When the ice melts, the glaciers drop their load of sand and pebbles forming a mound-like ridge of sediment on the landscape. If the glaciers carried a larger load of sediment, the mound would be higher.