One of [the] defining characteristics of [the northern Horn of Africa] is its separateness, and this is partly due to its physical diversity. A detailed description of physical geography falls outside the scope of this book. It may simply be noted that the region comprises highlands that are bounded on the east by the precipitous escarpment bordering the Danakil lowlands and the Red Sea. To the west, the country descends more gradually to the extensive plains of the Nile Valley but is riven by the rugged valleys of the Takezze and other Nile tributaries. In the north, with decreasing altitude, the terrain becomes progressively more arid as the Sudanese lowlands converge with the Red-Sea coast. It is only to the south that the highlands continue, linking them with the principal mass of the Ethiopian plateau, near the western edge of which lies Lake Tana and the source of the Blue Nile.