As the map is traversed, blocks are built of the screen edge. Each block has a hierarchy of content squares with descriptors, like:
(Tall obstruction, tree, ash, live)
As the block is left further behind, the descriptors drop off, yielding a smaller save file and a dynamic world. Eventually given enough time, what we a tree is now a light post. Add in a transformative option, so as a tree becomes a bush and cascades to differing new.
Peter's stuff.