Question: How many types of bedrock are in Manhattan?
About the article "Central Park", section "Geology".

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Colluvium filled bedrock hollows are the cause of many shallow earth landslides in steep mountainous terrain.
They can form as a U or a V shaped trough as local bedrock variations reveal areas in the bedrock which are more prone to weathering than other locations on the slope.
As the weathered bedrock turns to soil, there is a greater elevation difference between the soil level and the hard bedrock.
With the introduction of water and the thick soil, there is less cohesion and the soil flows out in a landslide.
With every landslide more bedrock is scoured out and the hollow becomes deeper.
After time, colluvium fills the hollow, and the sequence starts again. 
  1993: Bedrock - "For What You Dream Of" (Stress Records) (UK #25)  1997: Bedrock - "Set in Stone" / "Forbidden Zone" (Stress Records) (UK #71)  1999: Bedrock - "Heaven Scent" (Bedrock Records) (UK #35)  2000: Bedrock - "Voices" (Bedrock Records) (UK #44)  2001: Bedrock - "Beautiful Strange" (Bedrock Records)  2002: Bedrock - "Emerald" (Bedrock Records)  2003: Bedrock - "Forge" (Bedrock Breaks)  2005: Bedrock - "Santiago" (Bedrock Records)  2006: Bedrock - "Warung Beach" (Bedrock Records)  2007: John Digweed - "Gridlock" (Renaissance)  2009: John Digweed & Nick Muir - "Tangent" (Bedrock Records)  2010: John Digweed & Nick Muir - "Satellite / Meteor" (Bedrock Records)  2010: John Digweed & Nick Muir - "Satellite / Meteor (Remixes)" (Bedrock Records)  
Vernal pools can form anywhere that a depression fills with water.
They can be found on bedrock of many kinds, or in grasslands that form over a variety of soil types containing silts and clays.
They can develop hydric soils which are typical of flooded areas, including accumulations of organic matter, but this may not happen in drier areas.
In some cases there is a hard pan layer which causes the retention of water in the pools. 
Port Talbot has a variety of bedrock and drift types. 
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 Downtown and Midtown, 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.
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.
To the north, the Harlem River divides Manhattan Island from The 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 in area, long and wide, at its widest (near 14th Street).
New York County as a whole covers a total area of , of which are land and are water.
The northern segment of Upper Manhattan represents a geographic panhandle.
One neighborhood of New York County is contiguous with The Bronx.
Marble Hill at one time was part of Manhattan Island, 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 topography has been evened out.
Early in the 19th century, landfill was used to expand Lower Manhattan from the natural Hudson shoreline at Greenwich Street to West Street.
When building the World Trade Center, 1.2 million cubic yards (917,000 m³) of material was excavated from the site.
Rather than dumping the spoil at sea or in landfills, the fill material was used to expand the Manhattan shoreline across West Street, creating Battery Park City.
The result was a 700-foot (210-m) extension into the river, running six blocks or 1,484 feet (450 m), covering , providing a riverfront esplanade and over of parks.
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 the midtown district, dips down lower between 29th street and Canal street, then rises toward the surface again under the Financial district.
It has been widely believed this feature 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, as their foundations can be sunk more securely into solid bedrock.
However, new research and others have shown that economic factors played a bigger part in the locations of these skyscrapers.