Barbados lies on the boundary of the South American and the Caribbean Plates. The subduction of the South American plate beneath the Caribbean plate scrapes sediment from the South American plate and deposits it above the subduction zone forming an accretionary prism. The rate of this depositing of material allows Barbados to rise at a rate of about  per 1,000 years. This subduction means geologically the island is composed of coral roughly  thick, where reefs formed above the sediment. The land slopes in a series of "terraces" in the west and goes into an incline in the east. A large proportion of the island is circled by coral reefs.