In the interior of Canada an area larger than the province of Saskatchewan lies below sea level: Hudson Bay. Like a gigantic dinner plate, the land rises from the low interior gradually over hundreds of kilometers towards the margins of the continent. Much of the lowland is Canadian Shield, one of the foundation plates of the earth's crust, and it is a rugged, forested country of massive igneous and metamorphic rock, thin poor soils, many lakes, and memorable scenery.

We have mentioned how in the 18th century Sir Alexander Mackenzie recognized the great magnitude of the area we now know as the Shield. For many decades it was called the Laurentian Plateau because along the St. Lawrence River north of Quebec City its edge is abrupt; indeed the high ridges rising in the north, like a wall behind the river, are still called the Laurentide Mountains.

However, in 1883, the Austrian geologist Eduard Suess, in Face of the Earth, applied the term "shield" to the ancient nuclei of the continents because he thought the low stable plates resembled a warrior's shield laid flat on the ground. Gradually his term was adopted in Canada. Seen as a whole, the Canadian Shield in actuality had the reverse curvature of the profile envisaged in Suess's image, but a tough surface it certainly is. Of course, the edge of the Shield was well known along the St. Lawrence River even though it rebuffed settlement. In 1534 Jacques Cartier had already referred to it as "the Land God gave to Cain". Indians lived in the Shield, and fur traders got to know the terrain as they followed its myriad rivers and lakes to the fur bearing countries.

In the 1770s Samuel Hearne accompanied a band of Indians travelling along the northern edge of the forest from Churchill to the mouth of the Coppermine River on the Arctic Ocean, and we learn from him that it was rough but largely even country with no towering mountain barriers. It remained for two outstanding geological explorers, J.B. Tyrrell and A.P. Low to report on the rocks and map the northern mainland Shield in outstanding exploratory journeys made in the late 19th century.

The Shield has been one of the major challenges to Canadian geological study, because its rocks are billions of years old and have a complex history. Geologists now know that different parts of the Shield passed through periods of massive mountain building and subsequently were worn down to a fairly even surface. Geologists have divided the Shield into geological provinces based on the locations of former mountain systems. These mean little to present day travellers because the mountains are long since gone, completely eroded down to their roots, and the present surface cuts across the former systems. All this reinforces the impression that the rough wilderness terrain across half of Canada is fairly uniform in character.

For over half a billion years now the Shield has been relatively stable, though parts of it have been warped up or down. Downward movement permitted seas to invade the Shield and during those times younger strata were laid down. Most of the strata were probably stripped off again by erosion when the area rose above the surface again. At present the eastern margins on the Atlantic and along the St. Lawrence River are warped up 1200—1800 m forming the Torngats, Laurentides, and the mountains of Baffin Island. In Algonquin Park the rocks are domed into a gentle arch, and the north shore of Lake Superior is a high plateau extending eastward north of Lake Huron. Elsewhere there are local semi-mountainous areas, but these are very modest on a continental scale, so they do not distort the grand picture of evenness. Downward movements along fault lines of the earth's crust have created impressive linear valleys, for instance those followed by the Saguenay and Ottawa Rivers, and small blocks have been thrust up as in the Madawaska area.

Yet the present scenery owed its origin to another agency entirely: glaciation. Scratches on the rock show the direction of ice movement; loose material was stripped off, rock surfaces were smoothed and polished, and weak or jointed rocks removed. Much material was deposited as well, especially sands and clays in temporary glacial lakes in front of the retreating ice. Just such action produced the Great Clay Belt in northern Ontario and Quebec.

But ice reshapes only the surface, and the basic relief in the strong rocks of the Shield was not altered. However, surface erosion and deposition obliterated or at least deranged the previously existing drainage system. Shallow basins were gouged out and valleys blocked to produce countless lakes. Their presence helps give Canada the largest amount of fresh water in the world. Rapids-strewn streams which spill from one basin to the next make summer movement by land almost impossible. The canoes of the Indians, shallow of draft and easy to portage, were ideal for coping with such obstacles.

A great contrast to the Shield landscape is the Hudson Bay Lowland, a large area of nearly flat younger sedimentary rock underlying Hudson Bay and extending for many kilometers south of it. Since deglaciation there has been no time for a mature river system to develop on this gentle slope. Hence the Lowland is poorly drained. Much is wet land with trees sparsely scattered in bog, a terrain called muskeg. This kind of topography is found in many local depressions in the Shield, but the Hudson Bay Lowland is the largest tract like this in Canada, and it is sometimes called the Great Muskeg or Great Swamp. Belts of trees are only found on better drained land along the rivers, and the rest is muskeg, almost impossible to cross except when everything is frozen in winter. The great load of the continental ice sheets once depressed the crust, and now that the weight is gone the surface has been slowly rebounding. On the coast of Hudson Bay the land is still rising about one cm a year. Earlier uplift is clearly indicated by the many sand and gravel strand lines marking former shorelines, which are best seen on the Hudson Bay Lowland.

The Shield covers so much of interior high latitude Canada that this is where the northern limits of agriculture and tree growth are to be found. Farming is still possible in the Great Clay Belt of Ontario and Quebec, 250 km south of James Bay, but summers are shorter, productivity is less, and frost can be a hazard in every month. Here the limits of commercial agriculture are being reached, but forests still grow. A remarkable feature of the geography of Canada is the broad belt of boreal trees, composed of conifers and hardier deciduous trees, which sweeps in a great crescent across the Shield from Newfoundland to the Cordillera of British Columbia and the Yukon. Spruce, fir, pine and tamarack withstand the severe winters well, and a few broad leaved trees such as cedar, aspen and birch also survive. But the forests are under increasing climatic stress.

As we approach Hudson Bay and continue northward, longer hours of daylight in summer and of darkness in winter are experienced. Even with long daylight hours heat does not accumulate significantly in the low summer sun, so the air masses are cold. The cold air contains little moisture and in the stable atmospheric conditions there are few weather disturbances. Precipitation is thus so low that we might expect desert conditions, but evaporation is also low and as a result there is no shortage of water in the ground. The permafrost of course extends from the south shore of Hudson Bay northwards, but in spring the surface layer melts, perhaps to a depth of half a meter, permitting plant growth. However, to the north average temperatures fall drastically, and trees cannot survive when the growing season gets too short and winters too severe. Fur traders called the area beyond the boreal forest the Barren Grounds. The northward limit of survival for trees across Canada is called the Tree Line, though in fact it is a broad transition zone: the woods become more open, then there are stragglers a of diminishing size, and finally the trees die out completely. The Barren Grounds are only a part of the tundra vegetation zone which extends from the Tree Line to the northernmost reaches of Canada. Here there is a short burst of plant life in summer when the surface thaws and daylight hours are long. Dark lichens, sedges, heath and even dwarf shrubs in sheltered places, provide the background for a profusion of brilliantly coloured flowers, but these are quickly killed by frost and by September the land is dormant again. On windswept shores in the archipelago facing the Arctic Ocean conditions are so harsh that plant life in many localities is nonexistent and the surface is bare rock and loose stones.

For more photographs of the Canadian Shield click here