About 23,000 years ago, this place lay before a sloping wall of ice as two lobes of the Laurentide Ice Sheet, the Cape Cod Bay Lobe to the west and the South Channel Lobe to the east, came together here. By about 21,000 years ago, the ice had stopped its southward push and over time deposited rock, sand, silt, and clay, leaving after centuries of erosion only this fragile bluff. During glaciation, so much water was locked in ice that sea level stood about 400 feet lower than today, but as the ice melted, sea level rose. About 6,000 years ago, the ocean began flooding the coastal plain and eroding these glacial remnants, the backbones of Nantucket, Martha's Vineyard, and Cape Cod. Rising sea level and erosion of these glacial headlands left the treacherous shoals that figured in at least 750 documented shipwrecks and untold loss of life. Sankaty Head, itself a glacial remnant, later became an ideal place for a lighthouse: when Sankaty Light first shone in 1850 it stood about 280 feet from the bluff's edge, and when it was moved in October 2007 it was 68 feet away. With sea level still rising and erosion continuing, Nantucket's distant future remains uncertain.