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To the north, the Harlem River divides Manhattan Island from Bronx and the mainland United States. Several small islands are also part of the borough of Manhattan, including Randall’s Island, Wards Island, and Roosevelt Island in the East River, and Governors Island and Liberty Island to the south in New York Harbor. Manhattan Island is 22.7 square miles (59 km2) in area, 13.4 miles (21.6 km) long and 2.3 miles (3.7 km) wide, at its widest (near 14th Street). Manhattan is loosely divided into Downtown (Lower Manhattan), Midtown (Midtown Manhattan), and Uptown (Upper Manhattan), with Fifth Avenue dividing Manhattan’s east and west sides. Manhattan Island is bounded by the Hudson River to the west and the East River to the east. The bedrock underlying much of Manhattan is a mica schist known as Manhattan Schist. It is a strong, competent metamorphic rock created when Pangaea formed. It is well suited for the foundations of tall buildings and the two large concentrations of skyscrapers on the island occur in locations where the formation is close to the surface. In Central Park, outcrops of Manhattan Schist occur and Rat Rock is one rather large example.