Beyond The Gap by Harry Turtledove

Beyond the Gap (2007) is a Prehistorical Fantasy. The Glacier had been covering the northern plains forever. Only myths tell of a time before this mountain of ice. Legends also speak of a Golden Shrine -- sacred to the gods -- on the other side of the Glacier, but few believe these old tales. The Glacier had been retreating even before the founding of the old Raumsdalian Empire. Over the centuries, a gap in the great Glacier has been extending further north.