Interglaciation is the term used by geologists to refer to the alternating periods of warming and cooling in the earth’s past. The cooler times are called the “glacial period” during which ice shelves from the Arctic slowly creep southward and spread across the earth. Times when the earth is covered in these large ice sheets are known as glacial periods (or ice ages). When the ice sheets are not spread, it is called an interglacial period. The most recent glacial period occurred between about 120,000 and 11,500 years ago. Since then, the earth has been in an interglacial period called the Holocene.