Stone Spring: Stephen Baxter

Book I of Northland. Ana lives on the North Coast of Doggerland, a vast, fertile plain that linked the British Isles to Europe 10,000 years ago. Ana is fourteen. Her father is missing, her mother is dead. She has perhaps another 20 years of life left - life is short for her people even though they live in a veritable Eden teeming with wildlife, a world yet to feel the impact of man. But their world is changing. The ice has melted, the sea is rising and one fateful year a tsunami sweeps inland, scattering Ana's people. But if the people of distant Jericho could build a wall to keep the world out, surely Ana's people can build a wall to keep the sea out? Alternate history, set in a Mesolithic rich with alien cultures.