The most recent ice age reached its peak some 20,000 years ago with sea-levels some 400 feet (120 m) lower than today. For most of North America this added a ‘sleeve’ of additional land to its coastlines, but crucially, a landbridge was created over the Bering Sea, permitting the first human migration to the Americas. Bar the western coastal margins, Canada and much of northern America was then covered entirely by the vast Laurentide Ice Sheet, with today’s boreal and temperate forest zones transposed southward. After several thousand years of thaw, vast meltwater lakes formed. Climatic and vegetation zones shifted northwards, with northern forest colonizing much of the territory that had been encased in ice. In the Southwest deserts supplanted steppes, while tropical zones were scattered around the Gulf of Mexico, in what were once forested regions. Over the same period, mass extinction of the continent’s megafauna occurred, whether through human intervention, or the changes in climate/vegetation, or both, is disputed.
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