For thousands of years, northern Montana lay beneath massive ice sheets whose movement reshaped the region. About 190,000 years ago, glaciers advanced onto the northern Great Plains of Montana from the north and northeast, and because this area lay near the southern edge of the glacial ice sheet, the ice was thinner here than farther north. As the glacier moved south, it blocked the Missouri River and forced it into an ice-free channel to the south. The ice sheet lasted until about 130,000 years ago, when it melted away and left a radically changed landscape dotted with small ponds, glacial till, extensive gravel beds, and large boulders called erratics. Some of these erratics came from as far away as northern Manitoba or were torn from rock outcrops west of Lake Winnipeg. The Missouri River never returned to its original channel; the old course is now occupied by the Milk River between Havre and Wolf Point. About 25,000 years ago, the Wisconsin ice sheet pushed south into northeastern Montana, but highlands to the northwest diverted the glaciers to the east and west, leaving a large ice-free pocket between Glasgow and Hinsdale. The Wisconsin ice sheet retreated from Montana about 11,000 to 8,000 years ago. Between the Illinoian and Wisconsin ice sheets, mammoths, giant bison, and horses lived in this area.