Approximately 1.7 million years ago (the beginning of the Quaternary Period, Pleistocene epoch) geologists believe the plains of North America cooled. As a result, large ice sheets began to advance south from the Hudson Bay area of Canada and eastward from the Rocky Mountains. These ice sheets advanced and retreated many times toward the end of the Pleistocene epoch in intervals lasting from 10,000 to 100,000 years. This most recent ice age ended about 10,000 years ago, when the ice retreated to its present polar positions. Currently, the Earth is nearing the end of an interglacial (warmer) period, meaning that another ice age might be due in a few thousand years.