What caused virtually all of canada and some of the northern us to be once covered by thick glacial ice almost 15,000 years ago?

Changes in the earth's climate caused ice sheets to grow and cover most of Canada and some of the northern US almost 15 000 years ago. This is known as the last Ice Age. For the last two million years, the earth has shifted between ice ages and warmer periods. Ice ages are caused by slight changes in the way the earth orbits around the sun. Huge continental glaciers or sheets of ice moved down from the Artic and covered Canada, North America and much of Europe.