People of the Nightland (North America's Forgotten Past, #14)

It has been a thousand years since Wolf Dreamer lead his people up through the dark hole in the ice to a rich, untouched continent bursting with game. But the world has changed. Most of the magnificent animals are gone, and the last of the great glaciers is melting, forming a huge freshwater lake in the middle of the world. Over the centuries the People of the Wolf have split into two clans. The People of the Nightland live in the honeycomb of ice caves that skirt the glacier. The People of the Sunpath live in hide lodges to the south, hunting the few remaining mammoths, bison, giant sloths, and short-faced bear.
When a young orphaned boy named Silvertip receives a vision from Wolf Dreamer that their world is about to end, no one believes him--no one except a jaded war chief and a little girl. Led by Silvertip's dream, the three of them must convince both people to leave the land of their ancestors and flee eastward as fast as they can before the Ice Giants destroy the world.
This is the sweeping saga of a visionary boy who led his people out of the path of one of the worst catastrophes in the history of the world, and the brave little girl who loved him enough to believe in his dream.