冰河世紀的秘密

Over the Earth's long history， there have been a number of times when much of the northern hemisphere was covered by vast sheets of ice and snow. Such periods are known as ice ages. During ice ages， huge masses of slowly moving glacial ice- up to two kilometres thick-scoured the land like cosmic bulldozers. At the peak of the last glaciation， about 20，000 years ago， approximately 97% of Canada was covered by ice.