Pyramids in the Fifth Dynasty were no longer built of solid stone, but rubble-filled cores cased in limestone sheathing. This worked well enough until the Romans stripped the casing to burn for lime, exposing the rubble to erosion. Only worn hills remain. The Middle Kingdom pyramids were built of mud brick on limestone foundations, an engineering feat far more complex than designing the Old Kingdom stone models. These too have been stripped of their limestone casings, but much of the mud brick still remains.
