I think it is mostly sandstone, but the rocks are huge, giant boulders as big as cars. The rocks you see above ground in pictures of the lake, are the same rocks you see below the water. Large rocks are a little more difficult the aquarium. Also it depends on the species, since some live in the open water, some close to the rocks, some deep in the rocks and of course the sand dwellers. No plants to speak of a few in the shallows, but mainly rocks with a nice growth of green algae.