The Quaternary Period began about 2 million years ago. It is divided into the Pleistocene Epoch (2 million to 10,000 years ago) and the Holocene Epoch (10,000 years ago to present). Although 2 million years is a relatively short period of time geologically, sediments deposited during this time mask nearly all of Minnesota's previous geologic history (as old as 3600 million years!). The Pleistocene, also called the Ice Age, is the time when the Laurentide Ice Sheet covered much of northern North America. Minnesota was at the edge of the ice sheet, and thus was not always completely covered with ice. Instead, Minnesota experienced several episodes of glaciation followed by ice-free periods.