Alan Coombe. age 14, of Jamestown, N.Y., for his question:

WHAT IS THE CANADIAN SHIELD?

The Canadian Shield is a huge, rocky region that curves around Hudson Bay like a giant horseshoe. The Shield covers half the land area of Canada. It includes most of Baffin Island, all of Labrador, nine tenths of Quebec, over half of Ontario and Manitoba and large areas of Saskathewan and the Northwest Territories.

The Canadian Shield also dips into the United States to form the Adirondack Mountains of New York and the Superior Uplands of Michigan, Wisconsin and Minnesota.

The Shield is also called the Laurentian Plateau, after the Laurentian

Mountains of southern Quebec. _

Geologists estimate that the rock formations making up most of the Shield are 500 million to five billion years old. Most of the rocks have undergone one or more periods of mountain building. During these periods, extreme heat and pressure produce high mountains of granite, diorite, quartzite and other crystalline rocks.

Weathering and erosion wore down the mountains. Today, much of the Shield's central and northwestern part is fiat and low.

Relatively few people live in the region. Only small areas are suitable for agriculture.