Between the extinction of the dinosaurs sixty-five million years ago and the beginning of the Ice Age about two million years ago, the Age of Mammals unfolded in western Montana. During this time, geological activity in the western third of the state raised mountains and lowered valleys, and those valleys became home to a wide variety of unusual animals, many of which later went extinct or now survive only in much warmer climates. About thirty-five million years ago, Montana was home to double-horned, rhinoceros-like brontotheres, primitive little three-toed horses, camels, and deer. By about twenty million years ago, the valleys held strange ancestors of modern elephants, camels, deer, and pigs, including elephants with shovel-like tusks, deer-like animals with forked horns over their noses, and pigs as large as hippos. Montana was lush and subtropical, and these animal groups disappeared with the coming of the Ice Age and the arrival of humans less than fifty thousand years ago.