Twenty-five thousand years ago, after the last great continental glaciers had retreated, this area was a hummocky grassland dotted with ponds, swamps, and erratic boulders, with the Rocky Mountains to the west and the heavily glaciated Sweetgrass Hills far to the north. Its grasslands supported abundant animal life, including species still found on the northern Great Plains and many now-extinct animals that were generally larger and adapted to a colder post-glacial climate. Great herds of horses, pronghorn antelope, elk, camels, and giant bison shared the plains with Shasta ground sloths and shaggy musk and shrub oxen, while Columbian mammoths with long, curving tusks roamed in small groups. Dire and gray wolves and short-faced bears followed the herds, and saber-toothed cats and the larger, long-legged American lion were among the most formidable predators. Climate change, limited food supplies, and possibly overhunting by paleo-Indians contributed to the extinction of many species that had once been common on the northern Great Plains about 11,000 years ago, while American bison, gray wolf, elk, and pronghorn antelope survived as descendants of that earlier ecology. Shasta ground sloths had migrated to North America from South America about one million years ago, weighed between 300 and 400 pounds, were about nine feet long, and walked on their knuckles before disappearing nearly 11,000 years ago. About 11,000 years ago the first humans entered this game-rich landscape, and these early paleo-Indian hunter-gatherers became known for hunting mammoths and bison with spears tipped with Clovis and Folsom points. Many of these Pleistocene mammals, along with humans, are classified as megafauna because they were long-lived, had slow population growth rates, and had few or no natural predators capable of killing adults.