The map at left outlines in blue the extent of the Laurentide Ice Sheet 15,000 years ago. Geologists have found material left by the ice throughout this area, proving that virtually all of Canada and some of the northern US were once covered by thick glacial ice.
Similar findings show that ice was also widespread in many other parts of the world.
Scientists believe that changes in the earth's climate caused the ice sheets to grow. Learn more about Ice Ages from the Illinois State Museum.