Around 20,000 years ago, subarctic glaciers advanced southward across North America, and although they never reached the Smokies, the advancing glaciers led to colder mean annual temperatures and an increase in precipitation throughout the range. Trees were unable to survive at the higher elevations, and were replaced by tundra vegetation. Spruce-fir forests occupied the valleys and slopes below approximately . The persistent freezing and thawing during this period created the large blockfields that are often found at the base of large mountain slopes.