Where did all the rock on the National Mall come from? Locally quarried stone was used in the early part of Washington, DC’s history because it was too dificult to transport stone from other parts of the country. As railroads expanded in the late 1800s, the choices of building materials expanded as well. Suddenly, quarries from around the country were able to supply Washington, DC with rocks that showcased the unique geology of their region. Find out more about each type of rock used in the construction of the National Mall.
Igneous means “fire-formed” in Latin. Igneous rocks are formed when molten magma or lava cools. The magma can cool slowly or quickly, inside or outside of the earth’s crust. Magma that cools inside the crust forms intrusive igneous rocks. Lava, cooling outside of the crust, forms extrusive igneous rocks. Granite is an intrusive igneous rock.
Academy Black Granite
As dinosaurs roamed the earth during the Jurassic Age, magma at the base of ancient volcanoes in the Sierra Nevada foothills cooled to form this dark grey, almost black, intrusive igneous rock. It is now taken out of the earth from quarries near Raymond, California. Like other black “granites,” Academy Black Granite is technically somewhere between gabbro and diorite. Compare the Academy Black “Granite” benches with the true Carnelian Granite found at the FDR Memorial. You can also compare the unpolished FDR Memorial benches with the polished Academy Black wall at the Korean War Veterans Memorial.
Granite is an intrusive igneous rock. The Carnelian Granite that cooled inside the earth in the Early Proterozoic (2 billion years ago!) is now exposed at the earth’s surface in Milbank, South Dakota. The variety of colors and textures in each granite block at the FDR Memorial shows changes in the cooling speeds and chemistry of the magma that formed the Carnelian Granite. Some blocks have areas with tiny mineral crystals that cooled quickly. In most cases, interlocking crystals of clear quartz, pink feldspar, and various dark minerals cooled slowly, and are large enough to identify with your naked eye. Don’t be fooled if you see shimmers of gold in the granite…that’s just pyrite, also known as Fool’s Gold.
Indian Black Granite
The darkest rock possible was used for the Vietnam Veterans Memorial so that the polished surface would reflect like a mirror. The natural color of the Black Granite from Bangalore, India is the color inside the engraved names, not the color of the polished surface. Even so, this rock is much too dark to be called a true granite. Can you see big crystals of pink or gray feldspar and clear crystals of quartz inside this rock? If not, it is not really granite. Geologists give intrusive igneous rocks with different cooling speeds and magma chemistry different names. Gabbro is the geologic name for rocks with smaller and darker mineral crystals than granite. If the magma that formed this gabbro had cooled outside, instead of inside the earth, it would have been called basalt.
Rockland is a city located on the eastern coast of Maine where long rows of granite islands create a protective harbor. Glaciers once covered this area with ice over a mile thick. When the glaciers retreated, they scraped away all the softer rock and uncovered the granite. Have you ever tried to hold a beach ball underwater? It is buoyant, and wants to float to the surface. Well, land is buoyant too and without the weight of all that ice, the land was able to rise up very slowly. When glacier melt made sea levels rise, only the more resistant granite areas remained exposed as islands. Granite from quarries near Rockland, Maine is found inside the Washington Monument above the 150 foot level.
Milford Pink Granite
Historically, the town of Milford, Massachusetts was known for two things: boot making and granite. But well before people started making boots in the nineteenth century, the earth was busy making granite in the Precambrian Age. Milford Pink Granite is a popular building stone all over the world because of its subtle color and even texture. The colors in granite from Milford, Massachusetts range from pink to light gray to greenish gray, with black spots from the mineral biotite. In Washington, DC, look for Milford Pink Granite all around the base and plaza of the Lincoln Memorial.
Minnesota Black Granite
If intrusive igneous rock is magma that cools deep inside the earth, how does rock like this get to the earth’s surface? It either has to get pushed up, or everything above it needs to be worn away. Lots of very old intrusive igneous rocks are exposed in Minnesota at very low elevations because glaciers scraped off all the younger rock above them. St. Cloud, Minnesota got the nickname “Nitty Gritty Granite City” because of all the quarries there. There is even a popular county park and nature preserve called Quarry Park were you can explore (and swim in) several old quarry sites. The rock that supports the Thomas Jefferson Memorial statue does not show the large mineral crystals of true granite, but it formed in a very similar setting.
The rock used at the 56 Signers of the Declaration of Independence Memorial was supplied by the Cold Springs Granite Company. This company is very large, has branches in several states, and imports stone from around the world. Unfortunately, granite is a very common rock, and the name “Pink Granite” is not descriptive enough to figure out exactly where it came from. Taking a rock out of the ground and plopping it down somewhere else is like taking a note out of a symphony and playing it all by itself. It loses some of its meaning when it’s not in the right place. Yet with enough research and special tools, a geologist could study the texture, size, and chemical composition of the “Pink Granite’s” minerals to determine the exact rock formation from which the “Pink Granite” was quarried.
Over time, the bits and pieces of sediment held together by natural cement can become sedimentary rock. The sediment can be small like sand or big like boulders, and the cement can be very strong or incredibly weak. Sandstone, shale, and limestone are examples of sedimentary rock.
Limestone is a rock that forms in shallow ocean bottoms. In the Mississippian Age, most of North America was actually south of the equator and a warm, tropical sea covered the land from Nebraska to Pennsylvania. You are looking at fossils of crinoids, bryozoans, and brachiopods that lived over 300 million years ago when the shallow sea covered southern Indiana. When the sea creatures died and more sediment covered them, they eventually became part of the rock. The calcium in their bodies provides the natural cement that makes limestone a strong stone for construction.
In Washington, DC, look for Indiana Limestone inside the Lincoln Memorial, the Thomas Jefferson Memorial, and the outsides of the National Cathedral, Federal Triangle buildings, the Department of the Interior, National Theater, National Archives, Botanic Gardens, and the Pentagon.
Aquia Creek Sandstone
Sand, mud and gravel deposited over the last one hundred million years created the Atlantic Coastal Plain of North America. To a geologist, the Cretaceous Costal Plain rocks are very young. In fact, most of the sediments haven’t had enough time to become rock yet. The Aquia Creek Sandstone is one of the few Coastal Plain layers that has had enough pressure and cement to become a sedimentary rock.
Aquia Creek sandstone, quarried from Stafford County, Virginia was used in several Washington, D.C. buildings including the White House and Capitol. See this sandstone up close at the intersection of 15th Street and Constitution Avenue NW, where the Capitol gateposts and gatehouses stand.
To metamorphose means to change from one thing to another. Rock that undergoes some sort of change, usually from intense heat and /or pressure is called metamorphic rock. The minerals in the rock recrystalize to become something new. Marble and gneiss (metamorphosed granite) are examples of metamorphic rock.
Colorado Yule Marble
The Colorado Yule Marble was formed by contact metamorphism. The parent rock of the marble is a Mississippian-age sedimentary rock called the Leadville Limestone. The limestone was heated and pressurized by the intrusion of magma that formed the granite of Treasure Mountain dome during the Tertiary period to create the Colorado Yule marble.
In Washington, DC, Colorado Yule Marble is found at the Lincoln Memorial, the Tomb of the Unknown Soldier in Arlington National Cemetery, and in the Washington Monument as the official Colorado state commemorative stone.
The color and texture of marble can tell us about its past. Since marble is metamorphosed limestone, whatever starts out in the limestone will end up in the marble. If the limestone is composed of pure calcium carbonate, the marble will be pure white. The Alabama Marble has dark streaks in it because the parent rock was limestone mixed with other minerals. In order for light to shine through the Alabama Marble ceiling tiles at the Lincoln Memorial, each ¼ inch thick panel was treated with waxes. This made the white marble turn yellow and translucent, just like parchment paper. Have you ever noticed how white paper changes if it gets waxy or oily? Next time you eat pizza, look at what happens to your napkin when grease gets on it.
Deep sea deposits along North America’s ancient continental shelf were metamorphosed when the North American and African Plates collided. Today you can see the results of that collision in bands of marble that run throughout the Appalachian Mountains. In the mountains of northern Georgia, tightly interlocking crystals of pure calcium carbonate create an unusually white marble with no additional color veins. Compare the pure white Georgia Marble of the Lincoln Memorial statue with a slightly veined variety inside the Thomas Jefferson Memorial. And if you are in Pickens County the first weekend of October, visit the World's Largest Open Pit Marble Quarry during the Georgia Marble Festival to see the source for yourself.
There is something fishy about the rock that rings the Thomas Jefferson Memorial statue base. It is called Missouri Marble by the quarry, but it seems to have styolites and fossils that are more common in limestone than in marble. The heat and pressure involved with metamorphism usually destroys any fossils in the parent limestone. Plus, Missouri’s geologic history does not include many episodes of metamorphism needed to produce marble. This fine-grained multicolored rock is most likely limestone or dolostone, deposited during one of Missouri’s repeated floodings by shallow seas.
Pink Tennessee Marble
The light grey to dark pink colored stone quarried near Knoxville, Tennessee is from the Ordovician aged Holston Formation. It is part of eastern Tennessee’s folded and faulted sedimentary layers associated with the formation of the Appalacian Mountains. Dark squigly lines, called styolites, show where thin layers of mud and silt were squished between layers of limestone. There was enough pressure to somewhat recrystalize the limestone, but not enough to metaporpose it into marble. Technically, the Pink Tennesse “Marble” is still a sedimentary rock. The quarry calls the limestone “marble” because it gets shiny when pollished and is hard like marble. Look for Pink Tennessee Marble at the Lincoln Memorial, Thomas Jefferson Memorial, and the National Gallery of Art.
Plate Tectonics associated with the formation of the Appalachian Mountains caused incredible compressionary forces along the eastern edge of the North American Plate. Sedimentary and igneous rock layers recrystallized to become the metamorphic rock of the Piedmont. Quarries along Rock Creek, and at Little Falls, Maryland, provided Washington, DC with building stone from the Piedmont’s Sykesville Formation, also known as Potomac Bluestone (Gneiss). Foundations of the White House, Capitol, and Washington Monument, along with buildings like the Lock Keeper’s House and the Old Stone House in Georgetown were built with the Sykesville Formation.
Vermont Imperial Danby Marble
Marble from Danby, Vermont is known around the world for having a tight grain, white stone, and light veining. There are several color patterns found in Danby Marble, including gold, blue, and green. The Imperial Danby Marble, used in the District of Columbia of World War Memorial, Ulysses S. Grant Memorial, and exterior of the Thomas Jefferson Memorial, is white stone of Ordovician age with golden veins.
Washington Monument Marbles
There are marbles from three different quarries on the outside of the Washington Monument, but they represent only two different marble formations. Lee Marble from Lee, Massachusetts was used for only a few layers near the 150 foot level; the rest is built from the Precambrian Cockeysville Marble formation found near Baltimore. A quarry located in the town of Texas, Maryland supplied the first 150 feet of marble. Twenty-five years later, a quarry down the road one mile, in the town of Cockeysville, supplied the rest. Even though these two marbles have the same geologic name, there are some differences. The Cockeysville Marble from Texas is nearly pure coarse grained calcium carbonate, while the Cockeysville Marble from Cockeysville is finer grained with some magnesium.
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