Those were by the Mauryans successors. Most of the reliefs, cave art, etc, etc etc are coming from their successors ---->>> Sunga, Satavahana, Kharavela. More stuff survives from these guys. Thei cultures are very similar, so there's no harm in using references from those guys. Mauryans only made one rock cut cave we know of. And you can tell they were just starting to cut through rocks, because it's pretty modest. Stupas used stone or brick. If you see most of the structures behind the walls were made out of either wood or unbaked/baked brick with plaster over it.