The caves are carved out of flood basalt rock of a cliff, part of the Deccan Traps formed by successive volcanic eruptions at the end of the Cretaceous geological period. The rock is layered horizontally, and somewhat variable in quality. This variation within the rock layers required the artists to amend their carving methods and plans in places. The inhomogeneity in the rock has also led to cracks and collapses in the centuries that followed, as with the lost portico to cave 1. Excavation began by cutting a narrow tunnel at roof level, which was expanded downwards and outwards; as evidenced by some of the incomplete caves such as the partially-built "vihara" caves 21 through 24 and the abandoned incomplete cave 28.