Donna, Lalonde, Age 12, Of Campbellford, Ont., Canada, for her question:

Where is the Wisconsin Shield?

The pre cambrian rocks of the earth's crust were formed more than 500 million years ago, and most of them are deeply buried under layers and layers of more recent rocks. A geological shield is a place where the newer rocks have been shorn away and we get an outcropping of the ancient pre cambrian rocks.

The Canadian Shield centers around Hudson Bay through Canada, Minnesota and Wisconsin. Its ancient rocks were uplifted by mountains that have long since worn down to their roots. The uplands of northern Wisconsin are part of this vast formation, and we may call them the Wisconsin shield.