Exceptionally Brief Summary of the Geology of Manhattan:
For the most part, Manhattan is underlain by intensely folded and faulted metamorphic rocks
(see geological map). These rocks range in age from lower Paleozoic (Ordovician &
Cambrian; 500-545 Ma) to latest Proterozoic (545-1000Ma) and consist of three formations
(from oldest to youngest): the Fordham Gneiss, Inwood Marble, & Manhattan Schist.
Together they comprise the New York City Group. The gneiss represents the old, Grenvillian
basement (about 1000 Ma) of Laurentia (ancestral North America) whereas the schist and
marble were originally sedimentary rocks deposited on the seafloor of the ancient Iapetus
Ocean (the ocean which existed prior to the Atlantic). Radiometric cooling ages indicate that
most of the metamorphism is related to the Taconic orogeny (mid to late Ordovician) with
isotopic 'resetting' occurring during the Acadian orogeny (mid to late Devonian). The
topography of Manhattan is in large part controlled by the plunging brows and crests of
these three rock types. The Inwood Marble is the least resistent to weathering and forms
many of the valleys and lowlands on Manhattan (excepting the cross-structural lows
associated with NW-SE trending high-angle fault zones such as those along Dyckman and
125th Streets). In contrast, the Fordham Gneiss and Manhattan Schist are more resistant
and form ridges (e.g., Highbridge, Fordham, Riverdale). In fact, the structural culminations of
the Manhattan Schist in Midtown and Tribeca provide the substrate on which NYC's
skyscrapers are built.
Outline of the Geology of New York City
New York City has a relatively
complex history that dates back up
to a
billion years.
The only place that this history can
be studied is in the rocks the
underlie and support the city. The
city can be divided into 3 areas
where different rock types dominate.
Area 1 Manhattan and the Bronx
Area 2 Brooklyn and Queens
Area 3 Staten Island
Manhattan and the Bronx
Manhattan and the Bronx have the
oldest rocks in the City. In these
boroughs there are 3 major rock
types to be found--gneiss, marble,
and schist.
The gneiss, known as Fordham
Gneiss, is found mainly in the
Bronx. This is the oldest rock in new
York, with estimates up to a billion
years old. This rock is very resistant
to weathering, so it tends to make
up the ridges in the Bronx (Fordham Ridge, Highbridge and Riverdale). Because it is a
gneiss, it is characterized by light and dark banding. These bands are folded and bent up,
reflecting the high stresses this rock was once subjected to. Some of the major minerals are
orthoclase, biotite, hornblende, and quartz.
The marble is called the Inwood Marble and it is found in northeast Manhattan and in the
Bronx. It is almost pure calcite marble (with some sand) and it weathers rapidly. Because of
this, the Inwood Marble tends to make up the major valleys (such as below the Harlem
River) and flat-lying parts of these two boroughs. Examples can be seen in Inwood Park and
in the Harlem lowland (the flat area just east of City College). The marble is estimated to be
from either the Cambrian or the Ordovician period, or roughly 500 million years old. This
rock is characterized by its white color and by its lack of any foliation. The dominant mineral
is calcite, but there are some minor layers of mica and amphibole. There are also a few, rare
fossils, indicating that it was originally a limestone, but don't expect to find any.
The schist is the youngest of these 3 and it was also formed in the Cambrian or Ordovician
periods. It is found in what is called the Manhattan group. It is called a group because there
are at least two different kinds of schist present, an amphibole schist and a mica schist. All
the schists are characterized by a high degree of foliation, which tends to be almost vertical.
The amphibole schist contains primarily amphibole and because of this it develops fractures
or joints oriented at 60 and 120. These joints are the diagnostic features of a rather dull,
dark gray rock. The mica schist is characterized by a high degree of mica, whic gives it a
shiny, lustrous appearance. Foliation is also easier to see in the mica schist. One special
mineral found in the mica schist is garnet, a red, glassy mineral, which can grow quite large
and have gem quality. Usually, all you can see are little red grains.
The Manhattan group is found mainly in Manhattan, expecially below 96th St. where it is the
only type of rock. Like the Fordham Gneiss, it is a strong, durable rock that doesn't weather
too easily. the strength of this rock makes it possible to support the weight of Manhattan's
omnipresent, towering skyscrapers.
All 3 rocks are medium to high grade metamorphic rocks. They are also thought to be
metamorphosed from sedimentary rocks, based on their chemical contents. The gneiss was
probably originally a sandstone; the marble, a limestone; and the schist, a shale. Because
they were sediments from the sea, they were originally horizontal and relatively flat (law of
original horizontality). These rocks now are extremely folded up. How did this happen?
When moving plates force continents to collide, mountains are built, as with the Himalayas
today. The great forces involved in such collisions heat and deform the rocks substantially.
Some are melted completely, others metamorphosed, and others can be just folded up. This
is what happened on the east coast of North America in the Paleozoic era (570 to 225
million years ago). These collisions came in at least three distinct phases. First, some small
islands that no longer exist, then Europe, and finally Africa collided with North America,
building the Appalachian Mountains. The collisions took the rocks in New York that were
once a sea floor, buried them up to 15 km deep, heated them up to 600C and deformed
them at the same time. Then, the rocks that were above these (such as the igneous
Palisades sill consisting of diabase) were eroded away over many millions of years,
exposing the rocks we see today in the Bronx and Manhattan.
Brooklyn and Queens
In contrast to the geology of Manhattan and the Bronx, the material in Brooklyn and Queens
is unconsolidated or "loose" sands and silts. These deposits are also much younger than the
metamorphic rocks just mentioned. The ages range from 3 million to only 10,000 years old
at the surface and about 70 million years old below the surface, a vast difference with the
rocks of Manhattan and the Bronx.
The very young sands at the surface in Brooklyn and Queens are believed to be deposited
from glaciers. As the glaciers advanced from the north, they scraped up materials along the
way, like a bulldozer. When these glaciers retreated, the material that was scraped up was
left behind. This is how Brooklyn and Queens and in fact all of Long Island were formed. A
long ridge called a terminal "moraine" was left at the terminus of the glacier's advance, and
these ridges form the line of hills that run from Prospect Park in Brooklyn through Queens
and along the north shore of Long Island. Green Point and Montauk Point represent two
different terminal moraines, showing that the glaciers advanced more than once. Block
Island is also part of these moraines, but the regions between have been eroded away and
covered by the ocean.
The glaciers also left boulders on their path and smoothed and carved up the rocks they
passed over. The boulders are from distant parent rock bodies and are called "glacial
erratics". Examples can be seen in many parks in Manhattan, including Inwood Park, where
boulders from New Jersey sit atop the hills. Some of this glacial debris also filled up old
valleys and fault zones in the city. This debris is a weak material, so it can't support
skyscrapers. This is why you don't see skyscrapers between 23rd Street and Canal Street.
Summary
As you have just seen, the geology of New York is somewhat complicated and has a
venerable history. Hopefully with this lab and with your travels through the city you will gain
some insight and understanding of where we now live and work, and you can now go to
Inwood Park or Central Park and see mica schist that is 500 million years old, and possibly
even find tiny gems of garnet instead of just seeing black blobs of rock.