The Canadian Shield refers to the exposed portion of the continental crust underlying North America.
The Canadian Shield refers to the exposed portion of the continental crust underlying North America. The crust, also known as the North American Craton, consists of rock from the Archean and Proterozoic eons and extends from Mexico to Greenland. The Canadian portion runs from central Ontario north to the Arctic Archipelago, and from Labrador west to the Northwest Territories. The Shield includes some of the oldest rocks on Earth (possibly more than 4 billion years old). While at times a barrier to settlement, the Shield has also yielded great resources, including minerals, coniferous forests and the capacity for hydroelectric developments.
The Canadian Shield covers about 5 million km2. Repeated advances of glacial ice have scoured its surface and left it strewn with countless lakes, rivers, streams and ponds. Along its edge lie many of the great lakes and waterways of Canada: the eastern shores of Great Bear Lake, Great Slave Lake, Lake Athabasca and Lake Winnipeg; the northern shores of Lake of the Woods, Lake Superior and Lake Huron; and the north shore of the St Lawrence River.
The origin and age of the Shield were among the great mysteries of Canadian geology. Canada's oldest rocks (3.96 billion years) were found east of Great Bear Lake. The Shield's southern limits were traced by Alexander Murray, who, in 1851–52, examined the country below Gananoque, Ottawa, the St Lawrence and Ottawa rivers, and the perimeter from Kingston to Lake Superior. A.C. Lawson made an important contribution by working out the Precambrian succession in the 1880s, but a more current time scale was not developed until the 1950s, when geologists such as C.H. Stockwell had seismic and gravity measures at their disposal (see Geological History). Stockwell divided the Shield into three great provinces – Superior, Churchill and Grenville – and 23 sub-provinces. The word province was used to denote regions with similar geological histories. Today, geologists generally use the word “terrane” instead of province, and understand the Shield to be made up of pieces of the Earth’s crust which, through the process of plate tectonics, began colliding together more than 3 billion years ago (see Geological Regions).
The Shield has had a profound effect on Canadian history, settlement and economic development. In pre-European times it was the home of Algonquian nomadic hunters, who developed the birchbark canoe to travel its myriad waterways. The coureurs de bois, voyageurs and explorers used similar canoes to penetrate the continent.
The bare rock, thin soils, muskeg and insects of the Shield have presented a barrier to settlement. The Shield’s perimeter ends abruptly at the agricultural frontier of the Prairie provinces and eastern Canada. The railway link to the West literally had to be blasted through Shield rock, coincidentally exposing its great treasures: gold, silver, nickel, cobalt, zinc, copper and iron ore. Its coniferous forests and hydroelectric power support a large pulp and paper industry. Gigantic power developments at Churchill Falls, Labrador; James Bay, Québec; Kettle Rapids, Manitoba and elsewhere, feed electricity to the urban south.
This land of bedrock and bush has left its imprint on some of Canada's best literature, art and drama. For generations, the stark and rugged beauty of the Canadian Shield has attracted cottagers and recreationists from the urban south as well as tourists from all over the world.