The surface bedrock under the Miami area is called Miami oolite or Miami limestone. This bedrock is covered by a thin layer of soil, and is no more than 50 feet (15 m) thick. Miami limestone formed as the result of the drastic changes in sea level associated with recent glacial periods, or ice ages. Beginning some 130,000 years ago, the Sangamonian Stage raised sea levels to approximately 25 feet (8 m) above the current level. All of southern Florida was covered by a shallow sea. Several parallel lines of reef formed along the edge of the submerged Florida plateau, stretching from the present Miami area to what is now the Dry Tortugas. The area behind this reef line was in effect a large lagoon, and the Miami limestone formed throughout the area from the deposition of oolites and the shells of bryozoans. Starting about 100,000 years ago, the Wisconsin glaciation began lowering sea levels, exposing the floor of the lagoon. By 15,000 years ago, the sea level had dropped 300 to 350 feet (90 to 110 m) below the current level. The sea level rose quickly after that, stabilizing at the current level about 4,000 years ago, leaving the mainland of South Florida just above sea level.