Population: 7,322,564 (1990) Population density of 24,287 people per square mile or 9,377 per sq. km.

Physical Geography of Manhattan
Manhattan (Map) is really an island (see diagram below) surrounded by the Hudson River on the west and south, the East River on the east, and Harlem River on the north. All these rivers are really tidal estuaries.
Manhattan's substratum is comprised of layers of metamorphic rock (Fordham gneiss, softer Inwood marble, and tough Manhattan schist) scoured and shaped by glaciers as late as 10,000 years ago. Terminal glacial moraine covers much of the island. The edges are surrounded by mud banks.
The island is fairly flat. High and low points in Manhattan are 400 ft at Washington Heights and 5 ft at Battery Park. The total area of the city is 301.5 square miles or 780.9 sq. km. Some of this area has been created by landfill (notably, Battery Park City).
For many years, heights of the skyscrapers was determined on the underlying geology of the island, with the rocky substratum in the north part dipping deep below the surface moraine around 30^th Street and not resurfacing until below Lower Manhattan's financial district. Thus, few skyscrapers are found in these areas. With new building and landfill technology in the 1960's skyscrapers and Battery Park City were built on landfill.