The entrance to the Amenemhet III pyramid at Hawara. The pyramids of the 12th dynasty had an inner core made from mudbricks and an outer veneer made of limestone. Over the centuries, the outer limestone veneer has fallen away revealing the inner core which was made of mud bricks. There is also a lot of erosion. The mudbricks are likely to have been made by Israelite slaves who were in Egypt at the time. After the Exodus of the Israelites in the 13th dynasty (Neferhotep), no more large pyramids were constructed. Petrie found a workers village at Kahun in 1890. The artifacts he found in the village (namely the scarabs of Pharaohs) indicate that the village was constructed during the time of Sesostris II and was occupied until the 13th dynasty.
The Pharaohs of the Oppression - Israel in Egypt 