The Scandinavian chain-link shaft design changes into treelike branches where it enters the wheel; a reminder of the link between the stone monument and the tree of life. The oldest symbol of the world-axis (pole or link between heaven and earth) is the sacred tree, best known as the Tree of Life. Pillar stones began to replace sacred trees when nomadic peoples settled and cleared the land for agriculture. When the British Isles became Christian, these pillars were replaced with carved stone crosses.