but the Harlem River Ship Canal, dug in 1895 to improve navigation on the Harlem River, separated it from the remainder of Manhattan as an island between the Bronx and the remainder of Manhattan. Before World War I, the section of the original Harlem River channel separating Marble Hill from the Bronx was filled in, and Marble Hill became part of the mainland. Marble Hill is one example of how Manhattan's land has been considerably altered by human intervention. The borough has seen substantial land reclamation along its waterfronts since Dutch colonial times, and much of the natural variation in its topography has been evened out. Smaller islands Within New York Harbor, there are three smaller islands: Ellis Island, shared with New Jersey Governors Island Liberty Island Other smaller islands, in the East River, include (from north to south): Randalls and Wards Islands, joined by landfill Mill Rock Roosevelt Island U Thant Island (legally Belmont Island) Geology Bedrock The bedrock underlying much of Manhattan is a mica schist known as Manhattan schist of the Manhattan Prong physiographic region. It is a strong, competent metamorphic rock that was created when Pangaea formed. It is well suited for the foundations of tall buildings. In Central Park, outcrops of Manhattan schist occur and Rat Rock is one rather large example. Geologically, a predominant feature of the substrata of Manhattan is that the underlying bedrock base of the island rises considerably closer to the surface near Midtown Manhattan, dips down lower between 29th Street and Canal Street, then rises toward the surface again in Lower Manhattan. It has been widely believed that the depth to bedrock was the primary underlying reason for the clustering of skyscrapers in the Midtown and Financial District areas, and their absence over the intervening territory between these two areas. However, research has shown that economic factors played a bigger part in the locations of these skyscrapers. Updated seismic analysis According to the United States Geological Survey, an updated analysis of seismic hazard in July 2014 revealed a "slightly lower hazard for tall buildings" in Manhattan than previously assessed. Scientists estimated this lessened risk based upon a lower likelihood than previously thought of slow shaking near New York City, which would be more likely to cause damage to taller structures from an earthquake in the vicinity of the city. Locations Adjacent counties Bergen County, New Jersey—west and northwest Hudson County,