The Golden Shrine by Harry Turtledove

The glaciers came and covered the world with ice. Now they are in retreat. North of the city of Nidaros, north of the forest, north of the steppes where the nomadic Bizogots hunt, a gap has opened in the ice-wall. And down through that gap come the men who call themselves "Rulers." Their terrifying cavalry rides wooly mammoths. Their bows can shoot arrows farther than those of the southerners. Their wizards wield power that neither the shamans of the Bizogots nor the wizards of Raumsdalian Empire can match