Barbara Bell, “The Dark Ages in Ancient History: 1. The First Dark Age in Egypt” American Journal of Archaeology 75 [1971], pp. 1–26. Bell’s conclusions, based on analysis of textual evidence, have been supported by studies of past levels of the lakes that serve as sources of the Nile. See Karl Butzer, Early Hydraulic Civilization in Egypt (Chicago: Univ. of Chicago Press, 1976), pp. 28–33.