What Rocks on Your Block?
Building Stones Walking Tours
Introduction
Geology is everywhere.
The rocks and geologic features we live on and near play an important role in our natural history and have much to teach us. However, in urban environments it can be challenging to see a variety of rocks. Even though many cities, including New York, do have natural outcrops, these tend to be limited, and many city dwellers don’t have regular access to them.
That's where buildings come in. Even when we cannot easily go see geology, we can take advantage of the constructed world, which has brought geology to us. Many of the natural materials used to construct buildings have been shaped for people’s viewing pleasure. They give us access to a wide variety of rocks, fossils, and minerals without having to travel far.
These tours provide an example of how we can use the constructed world to teach geology and Earth science in our own communities. Although building stones lack their original context, they offer students excellent opportunities to practice their observational skills. Additionally, the tour allows us to connect the cross-cutting concepts of science, architecture, art, and history.
What Rocks are on YOUR block?
Teacher's Guide and Student worksheets are available at amnh.org.
Midtown East Walking Tour
Cement wall surrounding St. Catherine's Park
The material we view at this stop is in fact not a real rock at all! It is simply a cement wall that illustrates the concept of lithification among various quartz pebbles. In this particular example, the cement wall resembles a clastic sedimentary rock called conglomerate. Note the random distribution of rounded sediments of various sizes that are being held together.
Stone posts in front of 338 East 66th Street
These stone posts are an excellent example of granite that’s primarily composed of potassium feldspar. Easily distinguishable by its pink color, the potassium feldspar (as well as the other minerals in the example) formed large crystals due to the slow cooling process in which these rocks formed.
Marine fossils in limestone along Memorial Sloan Kettering Hospital building
This stop highlights an excellent example of an organic sedimentary rock. The building’s facade contains numerous examples that feature a wide variety of small marine fossils embedded within the rock. Most of the large intact fossils are molluscs, including gastropods (snails) and cephalopods.
Duane Reade store front
This stop on the tour provides an excellent example of mineral alignment (foliation) typically found within metamorphic rocks. Foliation occurs due to strong compressive forces while the ductile rock is cooling. In this sample of gneiss, the mineral grains are all aligned in a vertical orientation, which is perpendicular to the compressive force. Gneiss is easily identified due to its alternating light and dark banded layers.
Apartment building at 250 East 63rd Street
This location is another example of metamorphic rock. However, unlike in Stop 4, the foliation here is curved rather than straight. You can see many areas containing swirls, twists, and stretched minerals scattered across the building’s facade. These were formed during a time when extreme compressive and shearing forces acted on ductile rock. The green rock seen here is an excellent example of deformation.
Decorative Fill at 220 East 63rd Street
The small rocks you see at this station are igneous in composition. They are riddled with small holes. This vesicular texture forms when steam escapes from freshly ejected lava and extremely rapid cooling solidifies the rock. You can find further support for the rock’s rapid cooling by observing any crystals on its surface, which are small (or not there at all). Small-to-microscopic surface crystals identify this as an extrusive rock. Emerging from a hot environment into cool air, such rocks cool and solidify rapidly.
Building at 3rd Avenue at 62nd Street
This tour ends with a magnificent example of an intrusive igneous rock, gabbro (with the mineral bronzite). The large crystals indicate a longer period of cooling time, while the darker color indicates that the mineral composition is mafic—that is, the minerals contain significant iron and magnesium. The truly unique aspect of this station, however, is the eye-catching luster of the mineral crystals, which shimmer as you walk by. For the best effect, visit this stop in the early afternoon.
Fifth Avenue Walking Tour
Grand Army Plaza, near General Sherman Statue
There are three rocks used in this plaza: two granites and a bluestone. The first granite, the pink granite that makes up the statue base, is the Stony Creek Granite from Branford, Connecticut. This is a commonly used building stone throughout New York City. This granite has distinctively large crystals of a pink mineral—potassium feldspar—that is easy to recognize.
The second granite is found surrounding the statue, along the edge of the plaza, and as part of the benches. Notice that it does not have the same large potassium feldspar crystals, but it is still a granite. This is a good opportunity to talk about rock classification and point out that even though these two rocks may look different, they both still include quartz, plagioclase, and alkali feldspar, so they are granites.
The last rock here is referred to as Catskill bluestone by the industrial trade community. It’s very often used for the curbs and stones that line walkways around New York City. The rock itself is a type of sedimentary rock, sometimes also called a greywacke, that occurs in the Paleozoic sedimentary basins from the Catskill Delta of New York and Pennsylvania.
Building at 785 Fifth Avenue
Here there are two rock types: green serpentinite and white limestone.
The green rock is serpentinite, a metamorphic rock that formed when oceanic crust (basalt or gabbro) was altered by heat, pressure, and ocean water. These rocks typically form in subduction zones in the cores of mountain belts. Locally, there are examples of serpentinites on Staten Island and in Hoboken, New Jersey.
The serpentinite used here is called “verde antique” for its supposedly old, Roman look, but it is actually quarried in Vermont.
The white limestone is Indiana limestone, from what is formally called the Salem Formation, and is Mississippian in age (335 million years old). This rock is made up almost entirely of fossil fragments: predominantly bryozoans, crinoids, and foraminifera, but also many other shelly organisms. The facade of the Empire State Building is also made of Indiana limestone.
Harry Cipriani
Here we have a different kind of limestone. This limestone is from central France and is much younger than Indiana limestone. This one is Eocene, or between 33 and 54 million years old. The voids in this limestone are molds of whole, intact shells.
Delvaux at 781 5th Avenue
Here we see more varieties of rocks we have already seen at the other stops. The red rock above the door is another example of serpentine. This is a serpentine breccia and has been heavily altered by seawater. The red color here is from the presence of the mineral hematite, which is an iron oxide, Fe 2 O 3 .
We also see another example of a limestone, a type known as travertine, which typically forms from precipitation of carbonate out of freshwater such as lakes, rivers, or springs.
Plaza outside the GM Building and the Apple Store
This hard rock is an example of migmatite, a high-grade metamorphic rock. This rock contains quartz, feldspar, and biotite mica and is heavily folded. Look for examples of ductile folding as this rock formed under very high pressures and temperatures. This rock is what we call a partial melt because the temperatures were high enough to begin melting some of the minerals. This rock is from the 3-billion-year-old (3 Ga) Dharwar Craton in Andhra Pradesh, India.
The floor and benches here provide a great example of the difference between polished and unpolished surfaces. How do these compare?
GM Building
The columns outside the GM Building and inside the lobby are another example of carbonate rocks. These are marble, a slightly metamorphosed limestone. This coarse-grained marble, known as Georgia Marble, comes from Tate, Georgia (in Pickens County). This marble is world renowned for its purity and white color and has been used for the Lincoln Memorial and gravestones at Arlington National Cemetery.
Before leaving, look closely at the columns, and you may see a fun accessory mineral. This golden, cubic mineral is pyrite, or fool's gold. Pyrite is an iron sulfide mineral that formed during metamorphism of this rock during the building of Pangea around 380 million years ago.
9 West 57th Street
Here we see another example of calcium carbonate (CaCO 3 ), this time as travertine. Travertine forms from inorganic precipitation of calcium carbonate from freshwater, typically lakes, rivers, or hot springs, or as deposits in caves such as stalactites and stalagmites. A common feature of travertine is its large porosity, as you can see here—note the vertical holes and openings. The Colosseum in Rome is made of the same travertine!
Bergdorf Goodman
Here we see more examples of marble similar to what we saw at the GM building. Note the small (thumb-sized) pyrite, or fool’s gold, in the marbles.
Tiffany & Co
There are multiple kinds of rocks used here in this facade. The base is made of granite, with both white plagioclase feldspar, and pink alkali-rich feldspar. Note the difference between the polished and unpolished surfaces. Which surface type is easier to see individual minerals?
The walkway on the sidewalk beside this building is another type of granite, known as rapakivi granite, first described in Finland. The hallmark of rapakivi is its texture: large pink rounded alkali feldspars with a rim of white or grey plagioclase feldspar around it. These formed between 1 and 10 km deep in the Earth and this texture reflects slow cooling of magma at depth. Some of these stones have been replaced, but also by a similar rapakivi granite, except this one is a Precambrian granite from Quebec. (Can you tell the difference between the two granites?)
The windows are framed with a 3rd type of rock, one we have seen before a lot on this trip – limestone. This limestone is a breccia, a rock that has been broken apart and fragmented. Look closely to see if you can find any broken fossils. Crinoids are common in this rock.
Trump Tower
The dark-colored exterior rock around the doors of Trump Tower are a gabbro, an igneous rock that cooled slowly. This specific gabbro contains bronzite, a bronze-colored pyroxene mineral with sub-metallic luster.
Fifth Avenue Presbyterian
This church is made of the classic brownstone that was commonly used for buildings throughout New York City, especially in the 19th century. Brownstone is a sedimentary rock, primarily made of quartz and feldspar, sometimes called an arkose. This rock formed during the Jurassic period (~220 million years ago) after Pangea started breaking apart. As Pangea separated, a new basin was formed in this region near the areas that later became Hartford, Connecticut and Newark, New Jersey. Older rocks from Pangea were weathered, eroded, and redeposited into the lower-lying basins. The classic brownstones in New York City come primarily from quarries in Connecticut, where the rocks are slightly more durable than other nearby sedimentary rocks.
St. Patrick's Cathedral
St. Patrick’s Cathedral was originally built from blocks of a white marble quarried from Westchester County, called Inwood marble. Similar rocks extend south and can be seen in northern Manhattan and the southwest Bronx. Halfway through construction, the access and availability of these rocks changed, so the upper half of the Cathedral was built with a different, slightly grayer marble, leaving a distinct change in color. In recent years, restorations have obscured this division as older stones have been replaced with the gray marble.
Rockefeller Center, Atlas Statue
The base of the statue is carved out of Deer Isle granite, from Crotch Island, Maine. This granite is slightly unusual geologically for two reasons. The first has to do with the chemical composition of the large oval crystals of plagioclase feldspar seen in this rock. Plagioclase feldspar can have variable chemistry, being dominated by potassium (K), which is pinkish, or sodium (Na), which is whiter. Typically, it forms with a sodium-rich core and potassium-rich rim. Here, however, you see the pink, potassium-rich feldspar in the centers of the crystals, with white, sodium-rich rims.
The second unusual mineral in this granite is pyrite, an iron sulfide mineral that typically does not form in granites. Both unusual features tell us about the conditions under which this rock formed while it was cooling from magma inside Earth.
The walkway around the statue is made from two different rocks, both sedimentary: a blue-gray siltstone from the Catskills and the red-orange Crab Orchard sandstone from Tennessee. Note the red stained-looking pattern, which formed from iron interacting with water after this sandstone formed.
Rockefeller Center, Entryway
The columns on either side of the doorway behind the Atlas statue leading into Rockefeller Center’s International Building (45 Rockefeller Plaza/630 Fifth Avenue) are made of Indiana limestone on granite bases. The surface facing away from the building is rougher to the touch because it has weathered faster than the surface facing the building, which is more protected from rain and snow.
Rockefeller Center Plaza, Inside
The green walls along the hallway inside 45 Rockefeller Plaza (630 Fifth Avenue) are from a 150–200 million year old (Jurassic Period) metamorphic rock from Greece, known as Verde Tinos. Note the book-matched pattern of slabs used here. This architectural technique places slabs adjacent following the natural texture of the rock, creating a seamless, symmetrical appearance.
Rockefeller Plaza, International Building (45 Rockefeller Plaza/630 Fifth Avenue), Lower Level
The walls along the basement (both near the elevators and down the long hallway) are made of a fossil-rich limestone known as Crown Point limestone (from part of the Chazy Group on Isle La Motte in Lake Champlain, New York). The large spiral fossils are the gastropod Maclurites. Although they are similar in overall shell shape to ammonites, they lack sutures dividing the inside of the shell into different chambers. Maclurites fossils are more common along the hallway across from the restrooms. There is a nice nautiloid cephalopod next to the elevators.
Saks Fifth Avenue
The rocks used here are referred to as Ste. Genevieve golden vein marble by the building stone trade community. It is a weakly metamorphosed limestone (on the boundary between limestone and marble) from a quarry near the town of Ozara, in Ste. Genevieve County, Missouri, from the Devonian Grand Tower Formation. The key features of this rock are the abundant fossils, including solitary rugose corals, colonial rugose corals, and colonial tabulate corals.
Philippine Consulate
The facing stone is ignimbrite from the flanks of Mt. Pinatubo, Philippines. Ignimbrite is composed of a matrix of volcanic ash around rock fragments.
10 West 46th Street
The facing stone is larvikite, which is marketed as “blue pearl granite.” The rock is prized for its characteristic silver-blue schiller effect on polished surfaces, called labradorescence. (The schiller effect is metallic iridescence originating from below the surface of stone that occurs when light is reflected between layers of minerals).
16 West 46th Street
This is a slightly unusual rock known as a charnockite. It is similar to a granite but has a slightly different mineral composition. In addition to the normal feldspars and quartz found in a granite, charnockites also have pyroxene, a high temperature mineral not typically found with quartz. Charnockites tend to form in the deep cores of old mountain belts.
Amusingly, charnockite was first discovered as a new rock type in 1893 when Sir Thomas Holland, then the director of the Geological Survey of India, identified it from the tombstone of Job Charnock in the churchyard of St. John’s Church in Kolkata (then Calcutta).