It is very flat, rising to only 60 feet above sea level at a distance of 150 kilometres south of the Mediterranean coast, and is both intensely populated and cultivated. This plain is bound by 30 to 60 feet escarpments of raised Pleistocene deposits which are more continuous to the west than to the east where they are interrupted by the Wadi Tumilat valley near Ismailia. Along the coast, a series of large, shallow lagoons form an almost continuous belt from Alexandria to Port Said, the largest of which are El-Burullus and El-Manzala lakes. These lagoons are sites of intensive fishing activities including aquaculture. Discontinuous coastal dunes, 15 to 60 feet high and dune fields in the east, 15 to 30 feet high, constitute the only local relief. Offshore, the continental shelf of the Nile Delta is narrow and steep at Alexandria, but increases to 50 kilometres width east of the Rosetta mouth of the Nile, as far as North Sinai. The shelf break is at water depths between 260 to 600 feet.