Virtually all of Canada and some of the northern United States were covered by thick glacial ice about 21,000 years ago due to the last glacial period, specifically the advance of the Laurentide Ice Sheet. This massive ice sheet formed during the Pleistocene epoch and covered millions of square miles of northern North America, including Canada and the northern U.S., reaching up to 2 to 3 kilometers thick in places

. The cause of this extensive glaciation was a combination of climatic conditions during the last Ice Age, which involved long periods of cooler global temperatures. These conditions allowed ice sheets to grow and expand. The Laurentide Ice Sheet was the largest ice sheet in North America, and its growth was driven by the accumulation of snow and ice over thousands of years during colder periods when the Earth's climate was much cooler than today. This glaciation radically altered the geography of the region, carving out features like the Great Lakes and shaping the landscape with glacial deposits and erosion

. In summary, the thick glacial ice covering Canada and parts of the northern United States nearly 21,000 years ago was caused by the last glacial period's cold climate conditions, which allowed the Laurentide Ice Sheet to expand extensively across the continent