The Precambrian Shield is an extensive structural unit of the Earth's crust composed of exposed basement rocks formed during the Archean or Proterozoic eons which together comprise the Precambrian Era ending 544 million years ago. Originally formed during several rounds of mountain-building activity, Shield rocks are now among the most stable on Earth. The Precambrian mountain belts have since eroded away, creating the low, rolling rock plain we see today. The best-known examples are the Canadian Shield and the Baltic Shield in Scandinavia.

Canadian Shield

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, Bytown [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 3 great provinces - Superior, Churchill and Grenville - and 23 subprovinces. It is now considered to fall into 5 provinces: Bear, Churchill, Labrador, Southern and Grenville (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. Similar canoes were used by the COUREURS DE BOIS, VOYAGEURS and explorers to penetrate the continent.

Abundant fur-bearing animals in frontier regions provided the basis of a colonial economy until the early 1900s. The hegemony of MONTRÉAL was thus extended far into the wilderness, via the Ottawa River and connecting waterways into the North-West, creating the precedent for future Canadian sovereignty over these lands.

Barriers to Settlement

The bare rock, thin soils, MUSKEG and insects of the Shield have presented a barrier to settlement; the agricultural frontier of the prairie provinces and eastern Canada end abruptly at its perimeter. 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.