Nor was this all. No one looking at a map of the Delta can fail to have been struck by that extraordinary feature of the northern coast-line, the great lakes. At the present time there are in the Delta about 3,430,000 acres of land cultivated or under reclamation, and another 500,000 acres of waste land. North of these lie 1,180,000 acres, either permanently covered by the lakes or else flooded by them from September to December. Between the lakes and the sea is a belt of sand-dunes or sandy plains, pierced occasionally by openings. The sand-dunes are constantly being augmented by the prevailing north-west winds. These lakes or lagoons are for the most part extremely salt, and are distributed as follows, beginning from the west: Lake Mareotis, 70,000 acres; Edku, 60,000 acres; Borillos, 180,000 acres, and as much more during flood and early winter; and Lake Menzalah on the east, largest of all, 490,000 acres, and flooding 200,000 acres more at the same time of year. All these waste lands, now known as the Berea, were cultivated in Roman times, some being occupied by vineyards, others by wheat, and it would seem that the lakes were kept from extending landwards by dykes. But when the land was allowed to go out of cultivation no one had any interest in looking after the regulation of the lakes. First, the passage through the sand-dunes became silted up, because, as the basins decreased in number, less water was drained from them into the lakes at the time of the inundation. Then, after the closing of the openings, the water gradually rose, breached the neglected dykes, and completed the ruin of the land. Once the openings in the dunes are closed, the lake has to rise to a considerable height before it can force its way through again, owing to the continuous action of the sand driven by the wind. In this way it came to pass that neither of the lakes had more than one opening into the sea, and consequently, rising in flood-time above sea-level, invaded the lands to the south. Another cause may have possibly contributed to the same effect—namely, a sinking of the coast lands. Nowhere is this a more probable explanation of the facts than in the neighbourhood of Lake Manzalah. Before the Arab conquest much of what is now a shallow lake was famous for its gardens, palm-groves, vineyards, and wheat-fields; besides its agricultural villages, it contained towns famous for their cloth and cutlery manufactures, like Tunah, Damirah, Dabik, and, above all, Tinnis. But at the time of the conquest these towns were already islands in the lake, a position which enabled them to be the last stronghold of Coptic resistance to the Moslems.