The modern Danube Delta began forming after 4 000 B.C. in a gulf of the Black Sea, when the sea rose to its present level. A sandy barrier blocked the Danube gulf where the river initially built its delta. Upon filling the gulf with sediments, the delta advanced outside the barrier-blocked estuary after 3,500 B.C. building several successive lobes: the St. George I (3 500-1 600 B.C.), the Sulina (1 600-0 B.C.), the St. George II (0 B.C.-Present) and the Chilia or Kilia (1600 A.D.-Present). The Danube Delta is a low alluvial plain, mostly covered by wetlands and water. It consists of an intricate pattern of marshes, channels, streamlets and lakes. The average altitude is 0.52 m, with 20% of the territory below sea level, and more than half not exceeding one meter in altitude. Dunes on the most extensive strandplains of the delta (Letea and Caraorman strandplains) stand higher (12.4 m and 7 m respectively). The largest lakes are Dranov (21.7 km²), Roşu (14.5 km²), Gorgova (13.8 km²).