At high tide, the Chausey Islands Archipelago covers some 30 acres (10 ha) divided into 52 islands. The landscape changes drastically at low tide when nearly 4,942 acres (2,000 ha) of foreshore emerge in a mosaic of some 365 islands and islets. They are distributed within a rectangle approximately 4 miles (6.5 km) wide by 7,45 miles (12 km) long. Composed of blue granite for the most part, the islets have been largely punctured since the Middle Ages. Because of the rock's high quality, nearly 500 quarrymen worked on Grande Île in the 19th century.