THE EVOLUTION OF THE ROYAL TOMBS:Middle Kingdom Pyramids:

Hierarchical form was revived again in the Middle Kingdom, 12th dynasty of Pharaoh Amenemhet I. Pyramids were built with a core group of stone and framework of the pieces of raw, filled with a mixture of mud brick and rubble and sand. Limestone outer coating of fine tuff finish create an effect not unlike the stone pyramids of the Old Kingdom. However, the use of milk is a problem for the construction of the province, and once on the outer casing of limestone is to remove the pyramid has seriously deteriorated. As such, most of the Middle Kingdom pyramids today resemble little more than piles of rubble.