Glaciers
Objectives: Describe the two types of glaciers and features created by each.
A glacier is a large body of ice that slowly moves over the land. There are two types of glaciers:
valley and continental. Continental glaciers cover large areas of the land. Antarctica and Greenland are
covered by continental glaciers. Valley glaciers are usually long narrow glaciers that form in mountains.
They tend to form in mountain valleys. The sides of the valleys help to keep this type from spreading
out. The glaciers in Glacier National Park are this type.
During the Ice Ages, continental glaciers covered parts of North America including Illinois. There
have been three glacial periods that have covered parts of Illinois. They started about 300,000 years ago
and ended about 13,000 years ago. They are the Pre-Illinoian, the Illinoian and the Wisconsinian. The
Illinoian went as far south as near Carbondale. The Wisconsinian stopped between Paris and Marshall
and follows a line that goes south of Charleston through what is now Fox Ridge State Park. About 85%
of Illinois has been covered by glaciers. The glacial periods began quickly as summer temperatures were
7-13°F cooler and the winter snows did not completely melt. As snow accumulated, temperatures fell
more. In some areas of Illinois, the ice cumulated to 2,000 feet thick. The weight of the ice caused the
glacier to slide. As different glaciers formed and retreated, different types of plants grew and died
creating different soils.
Much of Illinois’s land features are due to glaciers. Much of the state’s low relief is due to
glaciers. Glaciers cause erosion by two methods: plucking and abrasion. Plucking occurs when pieces of
rock are picked up by the bottom of the glacier and carried away. As the rocks are carried along, they
can gouge and scratch the ground causing abrasion. As the sediments are carried along, they will
eventually be deposited producing landforms. The sediment carried by glaciers is called till. Many times
you will notice rocks scattered across plowed fields. The large pieces of till (rock) are called erratic. If
you begin to look at rocks along places like Big Creek or Lawrence Gravel, much of the rock is either
igneous or metamorphic. Since Illinois was not volcanic, these rocks had to get here somehow. The
rocks where brought down by glaciers from places as far away as Canada. The till that is deposited at
the edge of a glacier is called a moraine. It forms a ridge. The moraine at the farthest extent of the
glacier is called the terminal moraine. What is called Five Mile Hill south of Paris is the terminal moraine
of the Wisconsinian Glacier. It is the long hill you go up as you travel north on Rt 1. As the ice melted, a
large amount of water changed how streams and rivers flowed. The river channels become much wider.
You may have noticed that rivers are found in the middle of larger valleys. For example the Wabash
Valley is about 4 miles wide. The river is only a small part of it. At one time, the river filled the entire
valley. Along some rivers such as the Illinois River, there are large sand dunes created by sand being
carried away by the rushing water. The path the Illinois River takes was changed due to glaciers.
Farther north other glacial features become more obvious. One is a kettle lake. A kettle is a depression
in till filled with water after a large chunk of ice broke off a retreating glacier. Prairie potholes are
shallow lakes created as glaciers retreat leaving depressions due to uneven amounts of till when it was
deposited. Prairie potholes often contain water only in spring as snow melts fill them. These are very
important waterfowl nesting sites. The Great Lakes formed as large valleys carved by glaciers filled with
water.
Questions
1. Name and describe the 2 types of glaciers.
2. Describe how glaciers begin to form.
3. Describe plucking.
4. Describe abrasion
5. Define moraine
6. List and briefly describe 6 examples of the signs of glaciers in Illinois.