No, this is not a preview for another post-apocalyptic Hollywood survival movie. This catastrophe is a major part of Earth’s recent history and is best known for cavemen, mammoths and, well, ice. Lots of ice. It is less well-known as the engine of change in North American forests for the last two and a half million years: Forests were consumed, driven south, rearranged and reordered. As unfortunate as this sounds, without the ice age, the forests of today would not be as they are.