Gently sloping sedimentary rocks, their smooth lines occasionally broken by abrupt linear escarpments, overlap the much older Canadian Shield on its southern and western margins. Observant fur traders, including Sir Alexander Mackenzie, remarked on the succession of huge lakes arrayed in a line along what we recognize as the boundary between the Canadian Shield and the relatively easily eroded sedimentary platform. From Lakes Huron and Superior in the south they sweep in a great arc right across Canada through Lakes Winnipeg, Athabasca, Great Slave and Great Bear towards the Arctic, revealing by their presence where two great natural regions of Canada meet.

Along the Lower Great Lakes and the St. Lawrence River the sedimentary platform is relatively narrow, 300 km at its widest but usually much less, a low area snugly placed between the Shield and the Appalachians. From the Shield to the Rockies the sedimentary beds are much more extensive. The land is commonly thought to be level, but in fact it rises imperceptibly to the west over a distance of 1,150 km. Northward the sedimentary platform narrows to 150 km in the Mackenzie Basin, and 2,600 km from the 49th parallel the plains reach the Arctic Ocean.

Let us look first at Southern Ontario and Quebec. In travelling from the Shield to these sedimentary plains a remarkable transformation takes place as we go from rough forested lake-strewn terrain to gentle open farm countryside. It is not always thus, although in the 17th century Champlain had already noted the difference between the difficult country of the Shield and the pleasant lands to the south better suited for settlement. Great forests covered this country, and the trees approached those of the British Columbia coast in magnitude. But in the Lowlands of Quebec and Ontario over many generations the towering forests were cleared, making way for close settlement and the kinds of landscape shown in the paintings of Homer Watson.

These Lowlands began to be studied by geologists in the mid-19th century. The Geological Survey of Canada, under its great founder Sir William Logan, had little trouble in identifying the formations because European and American classifications of strata were readily applied to those rocks. However, there are differences between the surface features of the Ontario and Quebec Lowlands.

In Ontario the almost horizontal bedrock surface is broken by the abrupt wall of the Niagara Escarpment which crosses the Ontario peninsula from the Niagara River to Manitoulin Island in a sinuous line. Strong dolomitic limestone rocks at the top protect the weaker beds below. Many attractive waterfalls drop down the wall. The most magnificent of all, the 44 m Niagara Falls, down which the waters of the Upper Great Lakes pour on their way to Lake Ontario, has cut back 10.5 km from the edge of the Escarpment leaving the Niagara Gorge as evidence of the great erosive power of water. The surface of peninsular Ontario has been enormously affected by glaciation. A great accumulation of glacial till, much thicker than in the Appalachians and inherently more fertile because it is largely derived from limestone rocks, profoundly affects our perception of the landscape. Long sags and rises in the land, swelling into smoothly curved drumlins in some areas, have produced a soft undulating countryside, a land of easy gracious contours. Rough morainic ridges and flat former lake beds provide the element of contrast, but this is never a land of edges except where bedrock is exposed in the Escarpment. The rivers have cut deep ravines, but mainly they fit into the landscapes and do not organize it except where the Ottawa and St. Lawrence Rivers lead into Quebec. This expansive till surface underlying the well drained soils provided, when cleared, a magnificent base for agriculture. But the area also benefitted greatly from the presence of the Great Lakes which have become a vital part of the economic organization of the region.

In Quebec the Lowland is narrow, with the Laurentides and the Appalachians always visible as a backdrop. Life is concentrated on the plain. After deglaciation the lowland was covered by an arm of the ocean, the Champlain Sea, and the marine deposits produced a flatter surface than the tills of Ontario. The serenity of this plain is broken, not by a linear escarpment as in Ontario, but by a line of eight independent hills protruding abruptly 180 to 300 m above the flat floor along a distance of 80 km, each hill roughly round in outline and a few km² in area. These are the Monteregian Hills, possibly the necks of former volcanoes now eroded. Mount Royal in Montreal and Mount St. Bruno are the best known of these eminences.

If Ontario has been influenced by the Great Lakes, Quebec has been equally affected by the Great River. For centuries the St.Lawrence has been at the centre of life in Quebec, simultaneously providing splendid access to interior Canada and making possible the national transportation role of Montreal in Canada. The St. Lawrence River together with the Great Lakes form the greatest inland waterway in the world, shared in large part with the United States. The system is only broken by the rapids and falls in the St. Mary's, Niagara and St. Lawrence Rivers, all of which were overcome by canals and locks in mid-19th century.

Peninsular Ontario is a battleground for northern and southern air masses throughout the year, and the weather usually changes every few days to the beat of the air masses coming through. Precipitation is ample for agriculture and evenly distributed through the year. Much of the moisture comes from air masses originating in the Gulf of Mexico. In summer they can bring hot muggy tropical discomfort, with hardly any cooling at night. This is a continental climate and the winters are cold, but there are frequent mild spells as southern air moves in. The summers are the hottest in Canada. They are long enough, with a growing season of up to 220 days, and accumulate sufficient heat to permit corn to grow almost as well as in the American Middle West.

And the tempering effect of the lake waters keeps temperatures from dropping too low in winter along the lake shores so that peaches and grapes can be grown commercially on the coasts of Lakes Erie and Ontario, and apples more widely. In Eastern Ontario and the St. Lawrence Lowland of Quebec the heat accumulated in the warmer months is not as great as in the peninsula, so hay and pasture and mixed farming become more important. In October there may be a week or more of unseasonably warm hazy weather in southern Ontario and Quebec; this is the famous Indian Summer of North America.

Before settlement a mixed forest covered most of the Lowland. Only in the highly favoured area of hot summers and less chilling winters in southwestern Ontario did a fully mature deciduous forest flourish. In marked contrast to the boreal forest this area had an extremely rich variety of trees including elm, maple, oak, walnut, beech, chestnut, hickory and sassafras trees; there were also stands of huge white pine on light well drained soils. This deciduous forest and the adjacent mixed forest were largely destroyed in the 17th, 18th and 19th centuries in Ontario and Quebec and now the land is covered by farms and cities.

The Great Plains are on another scale entirely, and the fact that large sections are grasslands means that the essential flatness of the region could be appreciated at an early date and did not have to await the clearing of forests as in Ontario. The Indians and fur traders regarded the grassland as buffalo country, and recognized that the rim of woodland surrounding the prairie was an imortant natural boundary. It was the geologist James Hector of the Palliser exploring expedition who gave additional conceptual form to the southern plains so that people could more easily comprehend them. Hector identified the low area in present Manitoba next to the Canadian Shield, then the abrupt rise to the west to a higher level at what we know as the Manitoba Escarpment, next another rise farther west against the Missouri Coteau, and finally a gradual slope to the Rocky Mountain foothills. This concept of three prairie levels has been popularly accepted. Most of the westward lift is accomplished in the gentle incline of the three levels, Winnipeg at 240 m can be compared to Regina at 574 m and Calgary at 1079 m a.s.l.

Most of the surface of the Great Plains is till, but here it is much more spread out than in Ontario. Enormous flat clay deposits enhance the impression of overwhelming space; they include what is easily the largest level plain in North America, that of glacial Lake Agassiz in Manitoba. Variation is introduced by flattopped hills, remnants of a former plateau composed of weak rock, which stand out like islands in the plains, especially since they are forested. One of the residual plateaus, the Cypress Hills, higher than the others and located where the Alberta, Saskatchewan and U.S. borders meet, remained just above the ice sheets in the last glaciation and thus became a refuge area for plants. Other highly treasured contrasts to the general plains surface are the great valleys incised deeply into the plain by major rivers such as the Saskatchewan and Assiniboine. The Saskatchewan-Nelson River system drains the southern sedimentary plains into Hudson Bay, but the rapids-filled passage across the Shield is too difficult for use by any mode of transport except canoes or York boats. In the more arid sections of southern Saskatchewan and Alberta the soft banks of rivers unprotected by grass or trees have been minutely dissected by water into sharply gullied badlands. But mostly, the wide, flat-floored valleys are thinly wooded on the sides with meadows on the floodplains below, and may contain lakes as in the Qu'Appelle Valley.

Canada has two great treeless zones, the Barren Grounds we have already looked at, and the grassland areas of the Great Plains, located in an arid climate. As precipitation in the boreal forest region gets lower and as evaporation increases to the south, the woodlands become more open, and a transition zone of aspen copses and meadowland called the Park Belt, appears between the forest and the grassland.

Within the grassland itself sub-regions can be recognized. The fur trader David Thompson, writing in the 1840s, distinguished between the tall grass meadows next to the Park belt, often called prairies, and the short grass plains of the driest sections. Though the broad zones of forest and grassland are governed by climate, the exact location of the Park Belt may have been determined by early man's use of fire.

Within the plains there is a widely recognized gradient or decrease westwards from the Red River to the Rockies in both the amount and reliability of the precipitation. Adequate precipitation in summer and high day-time temperatures make southern Manitoba and southeastern Saskatchewan ideal for spring wheat growing, but in drier southern Alberta ranching and irrigation take over. Northward on the prairies there is enough precipitation for grain growing and mixed farming, but agriculture is limited by a 90 day dependable frost free season. The prairies have the most extreme continental climate in Canada. Winters are bitterly cold, but fortunately the weather is bright and crisp and snow remains firm underfoot. It is not the depth of the cold, but the length of the winter and the need to be constantly warmly dressed when outdoors which by late winter proves wearing on everyone. Blizzards are a hazard but can be coped with if people have the common sense to stay in and wait them out. In summer the days are blazing hot and the humidity is low, which produces high quality grain and is attractive to people who like the sun. The evenings and nights are cool, inviting outdoor activities and permitting restful sleep.

North of the Park Belt similar combinations of plains, low plateaus and valleys continue as in the south, but all are forested so that the contrasts are not as marked between the flat-topped erosion remnants of plateaus and the generally surface. In the Peace River Country a glacial lake plain, well drained by the magnificent Peace, is a fertile pocket of land. Mild Pacific air modifies the temperatures here so that the grain safely matures in northwestern Alberta. However, in much of the area drainage is so poor that muskeg abounds and prevents agriculture. The Athabasca River runs through sand areas saturated with oil, first noted by Peter Pond in 1788. These are now being exploited, and comprise Canada's greatest known reserve of oil.

As the sedimentary plains narrow to the north, another of Canada's master streams, the Mackenzie River, becomes the dominant feature. The Mackenzie drains into the Arctic Ocean the northern plains and part of the Shield, and through its tributaries, the Peace and Peel, it also drains part of the Cordillera. In summer, tugs and barges connect Great Slave Lake to the coast. Many optimistic claims used to be made concerning the potential for development of the Mackenzie Basin, but essentially the basin remains a corridor for movement to the Arctic where oil is produced at Norman Wells. North of Great Slave Lake permafrost is widespread and close to the coast it is continuous. Even when the surface melts in spring the sub-surface frozen layer impedes soil drainage and encourages soil slumping on even the slightest slope. As in all of Arctic Canada alternate freezing and thawing produces patterns in the ground such as polygons, and the Mackenzie Delta is famous for small hills looking like domes, called pingoes, formed of layers of ice and ground. Thus permafrost is a fragile environment where engineers have to take into account the summer surface thaw in any construction project.

In settling some of these lands man made tremendous changes, destroying forests and obliterating grasslands in the act of creating farmlands and cities. In this century we have recognized the need for stewardship of all the lands and waters of Canada. Not only are the unstable environments of the permafrost areas of the North now known to be vulnerable but we have been shocked to discover that the chemical pollutants produced in the everyday life of our industrial civilization are increasingly destroying vegetation and wildlife and contaminating our lakes and rivers. Two centuries after Mackenzie the face of nature in Canada is one we know well, but we also are learning that we alone can preserve its majestic beauty.