Manhattan is the most densely populated and smallest in area of the five boroughs of New York City. Located primarily on the island of Manhattan at the mouth of the Hudson River, the borough is conterminous with New York County, an original county of the state of New York. The borough and county consist of Manhattan Island and several small adjacent islands: Roosevelt Island, Randall's Island, Wards Island, Governors Island, Liberty Island, part of Ellis Island, Mill Rock, and U Thant Island; as well as Marble Hill, a small area on the mainland bordering the Bronx. The original city of New York began at the southern end of Manhattan, expanded northward, and then between 1874 and 1898, annexed land from surrounding counties.