In Avalon's tree-world, a knothole becomes a valley, and the outside of the trunk becomes similar to a cliff, or to Amara, though Avalon's surface is less steep than Amara's.
The passage describes a world where trees have unique features. A hole in the tree creates a valley, and the exterior of the tree resembles a cliff, similar to another place called Amara, but Avalon's surface is gentler than Amara's.