The Canadian Shield

It was the eminent Austrian geologist Eduard Suess who, in 1883, named the famous rock formation at Canada's heart the Canadian Shield. In choosing this name, he wanted to suggest the dome shape that he ascribed to it. However, we now know that the Shield is instead bowl-shaped, with a central depression occupied by Hudson Bay. On the periphery, escarpments rise to approximately 2,100 metres on Baffin Island and 1,600 metres in Labrador. Along the southern and western boundaries, heights approach 1,000 metres. Everywhere else, the Shield has the appearance of a vast plateau, with heights generally varying between 300 and 500 metres.

	Aerial view, Great Slave Lake, N.W.T. ©2003 ICN-RCI / Hemera

"We are living on the largest shield in the world/ and our rock crumbles over the ages/ one particle at a time/ in the imperceptible expectation/ of a time quake." [Translation] Poetry and geology merge in these few lines by the Franco-Ontarian author Agnès Whitfield. The Canadian Shield, which forms the stable core of the North American continent, is composed of the most ancient rocks on the planet. In 1986, one of the oldest rocks ever identified was discovered northwest of Great Slave Lake. It is thought to be 3.96 billion years old, a respectable age considering that the earth itself is 4.6 billion years old. These formations, consisting of volcanic rocks and rocks altered by intense pressure and heat, bear minerals such as iron, nickel, copper, zinc, uranium, magnesium and lead, to say nothing of precious metals such as gold, silver and platinum, which are an inestimable treasure for Canada. In 2001, for example, Canada produced 27 million tonnes of iron ore, 1 million of zinc, 614,000 of copper, 184,000 of nickel and 150,000 of lead.

The Shield owes its present-day appearance to both a very long period of erosion, which has worn down contours that were once spectacular, and much more recent events, which occurred during the last glaciation. Like monstrous machines, the ice sheets literally scraped and denuded the Shield, pushing loose soil ever farther in front of them. It is not surprising, then, that this region is today so ill suited to farming.

The glaciers also greatly disrupted the watercourses, filling a creek bed here and carving a canal there. These upheavals explain the young and somewhat erratic character of the current hydrographic network, with its multitude of lakes and swamps and its series of falls and rapids.