Brooklyn College

Core Curriculum 3.32

Geology

The Science of Our World
GEOLOGY OF NEW YORK CITY
Local geography
Locate the following geographic features and indicate their positions by placing the indicated abbreviations on the map above:
Upstate New York (UNY) New Jersey (NJ) Connecticut (CT)

Atlantic Ocean (A)

Hudson River (HR)

Long Island Sound (LIS)

Nassau County (NC)

Brooklyn (BK)

Queens (Q)

Suffolk County (SC)

Westchester (W)

Bronx (BX)

Manhattan (M)

Staten Island (SI)

New Jersey (NJ)

Hudson River (HR)

 

Jamaica Bay (JB)
 
More detailed map of the geography of NYC
Brooklyn (BK) Bronx (BX) Manhattan (M)
Queens (Q) Staten Island (SI) Westchester (W)
Coney Island (CI) Rockaway (R) New Jersey (NJ)
Hudson River (HR) East River (ER) Harlem River (HR)
Jamaica Bay (JB) Long Island Sound (LI)
Using the figures above, list the four counties on Long Island from west to east:
________________ _______________ ________________ _______________
Using the figures above, list the five boroughs of New York City:
____________ ____________ ____________ ____________ ____________
Which is the only borough of N.Y.C. that is part of the mainland of North America?
______________________
Local Topography
Topography is the description of the configuration of the surface of the land. Including the sizes and shapes and elevations above sea level of such features as hills, valleys, cliffs and plains. The figure above includes a topographic map of Brooklyn, Queens and western Nassau County, together with a schematic block diagram that has the north south profile along line A-B as its western face. Profile A-B runs from point A on the north shore of Queens to point B on the south shore of Rockaway. For the sake of clarity, vertical distances in the profile have been exaggerated. On the map, heights above sea level are indicated by the use of shading, as indicated in the legend. Other areas are shown in outline but without their topography.
What is the vertical exaggeration of the profile? (Hint: Vertical exaggeration is vertical scale (shown in the profile) divided by horizontal scale (shown on the map). Before the division is performed, the two scales must be expressed in the same units. Here, the vertical scale is shown in feet, the horizontal scale in miles.) Show your calculations and answer in the space below.

The vertical exaggeration is ___________ times.

Based on the figure above, briefly describe and contrast the topography of the northern parts of Brooklyn and Queens as opposed to the southern parts of these two boroughs.

Northern:
_______________________________________________________________

Southern:
_______________________________________________________________

The figure above is a simple topographic map of Manhattan, the Bronx, and the Palisades of New Jersey. Elevations of land surface are shown by patterns indicated in the legend. Waterways are white. Note that with the exception of mid- and lower Manhattan, much of the region is characterized by a series of elongated hills (called ridges) and valleys that are roughly parallel to (a) each other, (b) to the length of Manhattan, and (c) to the Hudson and parts of the Harlem and East Rivers.
What is the approximate compass quadrant bearing of these features? ______________
What is their azimuth? __________

The Palisades
As seen above, an explanation for the topography of Manhattan and the Bronx may be found in variable resistance of bedrock to erosion. A similar control of landscape by variable resistance of bedrock has been proposed for the west bank of the Hudson River. This control is called "differential erosion." There, the Palisades is an elongate ridge that parallels the shore of the river. The east side of the ridge is a sheer cliff that rises hundreds of feet. The west side of the ridge slopes gently downward to the west. The topographic profile X-Y, shown below, illustrates the asymmetric character of the Palisades ridge and also shows details of the topography of northern Manhattan and the South Bronx.


In the figure above, the locations of the Hudson and Harlem Rivers are shown. Which letter (a, b or c) indicates the position along the profile of the following three other important features:
The ridge of northern Manhattan __________
The Palisades ridge __________
The ridges and valleys of the southern Bronx __________
Local Geology

 

The figure above shows a generalized geologic map of the New York region that shows the distribution of different rock and sediment types that underlies the area.

Examine the map above that shows the location of rock samples.

Sample from location 1

Name of the rock___________

Is it this rock sedimentary, igneous or metamorphic? _______________

Hardness of the rock (soft, intermediate or hard) ________________

Reacts to HCl (hydrochloric acid)? _____

Sample from location 2

Name of the rock___________

Is it this rock sedimentary, igneous or metamorphic? _______________

Hardness of the rock (soft, intermediate or hard) ________________

Reacts to HCl (hydrochloric acid)? _____

Sample from location 3

Name of the rock___________

Is it this rock sedimentary, igneous or metamorphic? _______________

Hardness of the rock (soft, intermediate or hard) ________________

Reacts to HCl (hydrochloric acid)? _____

Sample from location 4

Name of the rock___________

Is it this rock sedimentary, igneous or metamorphic? _______________

Hardness of the rock (soft, intermediate or hard) ________________

Reacts to HCl (hydrochloric acid)? _____

Sample from location 5

Name of the rock___________

Is it this rock sedimentary, igneous or metamorphic? _______________

Hardness of the rock (soft, intermediate or hard) ________________

Reacts to HCl (hydrochloric acid)? _____

Sample from location 6

Name of the rock___________

Is it this rock sedimentary, igneous or metamorphic? _______________

Hardness of the rock (soft, intermediate or hard) ________________

Reacts to HCl (hydrochloric acid)? _____

Sample from location 7

Name of the rock___________

Is it this rock sedimentary, igneous or metamorphic? _______________

Hardness of the rock (soft, intermediate or hard) ________________

Reacts to HCl (hydrochloric acid)? _____
Differential erosion

Imagine flat bedrock that has not yet undergone weathering and erosion. The bedrock consists of two types: one with low resistance to weathering and erosion surrounded by another with high resistance. See figure above.

After a period of weathering and erosion, how would the elevation of the land surface overlying the low resistance bedrock compare with the elevation of the land surface overlying resistant bedrock?

The low resistance bedrock would be: Higher or Lower (Circle One).

The landscape feature underlain by the low resistance bedrock would be called a: Ridge or Valley (Circle One).

 

Block diagram showing the topography and bedrock geology of northern Manhattan, the western Bronx, and the Palisades of adjacent New Jersey. See how the ridges are underlain by a hard rock, whereas the valleys (rivers) were carved on the softer rock.
Brooklyn and Queens

In most areas of Manhattan and the Bronx, bedrock is at or close to the surface of the earth. In Brooklyn and Queens, bedrock is buried by significant thickness of sediment. The depth of the bedrock in these boroughs may be seen in the figure above. This figure shows the generalized north-south geologic section and topographic profile across western Long Island, showing locations at which samples were collected.

Because bedrock is not exposed at the surface, hypotheses other than differential erosion of bedrock must be sought to explain the landscape. Three major landscape features occur:
1. Hills that dominate the northern parts of the borough.
2. A low, flat plain that slopes gently southward from the foot of the hills towards the south shore of Long Island.
3. A series of narrow beaches and dunes that run along the south shore.

Sample from location 8

Do you see layering? ________

What color are the sediments? _______

What is the grain size? _______

Is there uniformity of grain size? (sorted/unsorted)

Regarding composition, do you observe one mineral, several or many? ______

Sample from location 9

Do you see layering? ________

What color are the sediments? _______

What is the grain size? _______

Is there uniformity of grain size? (sorted/unsorted)

Regarding composition, do you observe one mineral, several or many? ______

Sample from location 10

Do you see layering? ________

What color are the sediments? _______

What is the grain size? _______

Is there uniformity of grain size? (sorted/unsorted)

Regarding composition, do you observe one mineral, several or many? ______

Sample from location 11

Do you see layering? ________

What color are the sediments? _______

What is the grain size? _______

Is there uniformity of grain size? (sorted/unsorted)

Regarding composition, do you observe one mineral, several or many? ______

Explain the differences between the beach and dune sediment in terms of their origin.

What geologic process is most likely to have deposited the sediment out of which the plain is made?
Justification?
Boulders

Boulders made of granites, pegmatites, basalt, sandstone, etc. These boulders are distributed throughout the five boroughs: Manhattan, Bronx, Brooklyn, Queens and Staten Island. In those boroughs, some (but not all) of the boulders are found at the very tops of hills. The boulder on the right is made of pegmatite an intrusive igneous rock. Where, with respect to the Hudson River, is bedrock of the same compositions as the boulder on the right? _________________

The boulders in the hill sediments must represent pieces of bedrock separated from the bedrock and transported to their present sites in the northern hills of Brooklyn and Queens. What geologic agent is capable of moving boulders of this size for several miles?
Geologic Agent: Wind Flowing Water Glacier Gravity (Circle One)

Is this geologic agent at work in this region today? Yes No (Circle One)

 

The pegmatite on the right makes up the boulder shown above.

Why is bedrock at or near the surface in Manhattan and the Bronx? _____________________


Why are the boulders (and the rest of the sediment mix in which they occur) present in northern Brooklyn and Queens? Why is this area higher and hillier than the area to the south? __________________


What is the origin of the flat plain in southern Brooklyn and Queens? Does this theory of origin fit in with your earlier idea as to the agent that deposited the sediment in this area? __________________________