A country born of the ice Canada emerged from the ice age a mere 10,000 years ago at the onset of a warming period that is still with us today. That ice age was by no means the first. Over the last 900,000 years, there has been a succession of glacial periods, each lasting about 100,000 years, interspersed with brief warm spells less than 20,000 years long. Not all scientists agree on the causes of these ice ages, but more and more of them believe that cyclical variations in the earth's rotation and its orbit around the sun were to blame. The most recent glaciation began about 100,000 years ago. The climate became colder. The snow didn't melt completely during the summer and started to pile up. Gradually, glaciers formed and grew into a gigantic ice cap as much as three to five kilometres thick. The slow advance of the ice was quite uneven; there were times when it stopped, or even retreated. Some 18,000 years ago, the ice sheet reached its maximum expanse, covering nearly all of Canada's land mass. Only a few fortunate places escaped the glacial onslaught: high mountain peaks, coastlines and, in particular, a huge portion of the continent's northwest corner, whose arid climate prevented the snow from accumulating. This region, which extended from Alaska to the Northwest Territories, was linked to Asia for thousands of years by a land bridge that is now submerged beneath the Bering Strait. Consisting of tundra and sparse forests of spruce, the area was teeming with life. It was truly a crossroads, allowing plants and animals to move from one continent to the other, and human beings to make their way to the Americas. Even today, the Bluefish and Old Crow valleys in the Yukon hold the remains of a remarkable variety of prehistoric fauna: woolly mammoths, American mastodons, giant beavers as large as modern bears, sabre-toothed tigers, American lions and steppe bison. Unfortunately, none of these extraordinary animals survived the warmer periods that began about 17,000 years ago.
While the cold had set in almost imperceptibly, the warming occurred much more quickly. The ice sheets that had taken tens of thousands of years to build up melted away in about 12,000 years. The Cordilleran glaciers have been their present size for some 10,000 years, while the glaciers in northern Quebec and Labrador vanished completely 6,500 years ago. They left behind enormous lakes fed by meltwater. Furthermore, the sea invaded the areas that had sunk under the weight of the glaciers, such as the St. Lawrence Valley and the Hudson Bay Lowlands. As the bedrock rose and a new hydrological system evolved, the geography of Canada as we know it today began to take shape.
