Welcome to the National Mall, a National Park in Washington, DC where large stone monuments and memorials honor important historical people and events. The National Mall is a good place to visit if you want to learn about American history and be a historian. Because of all the different stones used in the construction of the memorials, it is also a good place to visit if you want to learn about rocks and be a geologist.
Historians and Geologists actually have many similarities. They both look at past events to better understand the present, and guess what will happen in the future. They both use tools to help them in their research. They both make timelines to keep track of events. The biggest difference is that Historians study the events of humans while Geologists study the events of the earth. The history of humans spans about 100,000 years, while the history of the earth goes back as far as 4.6 billion years.
Sometimes it is difficult to imagine how events that took place millions of years ago can still impact our lives today. Yet geology influences history all the time. What happens if we look at Washington, DC's geology and history together? We can call it "Geo-story" (because that sure sounds a lot better than Histology) and find out how the rocks help to support and tell the story of our country.
Millions of years of geologic events came together to produce a suitable place for Washington, DC to be developed. Geology shapes the earth's different rock types into mountains, valleys, plains, oceans, and everything in-between. The shape of the land and its relationship to water influences weather, which in turn influences the kinds of plants and animals that can survive. Also, certain types of rock are sturdy enouth to build on while others are not. So, geology creates the natural foundation that can support or prohibit human settlement. Settlement and development impact the history of an area as new people and ideas interact and natural habitats and landscapes change. Once we understand the impact of geology on an area, we can see how the scenery, development, and history all come together.
In downtown Washington, DC we have not only the geology under our feet to explore, but also the geology of the memorials which are made from rocks that have been gathered from around the country. These stone memorials help to tell the stories of America's past. Have you ever wondered about the stories of the stones themselves? Rock types used in each memorial were chosen for their unique color, texture, and strength as well as the moods and stories they create. The stone not only provides the building blocks of each structure, but strengthens the themes and ideas of the monuments and memorials as well. If you look only at the art and architecture of each site you will miss the great variety of rock "outcrops" in the middle of Washington, DC. But if you look only at the rock, you miss the great stories and meanings of the memorials. Why not look at both at the same time? Go ahead, be both a historian and a geologist as we discover the Geo-story of the National Mall.
From hills and valleys to mountains and beaches, geology has left its impact on the Washington, DC area by shaping the diverse terrains found within it. Soil and vegetation may cover much of the rock at the earth's surface here, but that cannot hide the story below. The geologic history of this area is a combination of unique and complex events that include plate tectonics, erosion and deposition, cataclysmic impact, and sea level fluctuation. Luckily, there is a way to make these events simple to remember. The I.M.P.A.C.T. of geology on Washington, DC is easy to remember when you Imagine the Movement of Plates that created both the Appalachian Mountains and the Atlantic Ocean basin along with the Crater from the Chesapeake Bay meteorite that helped shape the Washington, DC area over Time.
The earth is not one solid rock. It has molten magma on the inside and a thin, solid crust on the outside. The crust is broken into pieces, or plates, that move around on the molten magma, like ice on a lake. The first three letters of I.M.P.A.C.T. focus on plate tectonics (Imagine the Movement of Plates). The continental and oceanic crust that makes up the earth's surface is actually broken up into more than a dozen individual moving pieces called plates. As the plates move around they bump into each other (convergent plate boundary), slide past one another (transform fault boundary), or break away from each other (divergent plate boundary). The speed of plate movement is on average the speed of human finger- nail growth, but the consequences of the "bumps, slides, and breaks" are what cause volcanic eruptions, mountain building, earthquakes, and the creation of new crust. Geologists look at rock and fossil records to determine the placement and movement of different plates through time. Have you ever noticed that some modern continents look like puzzle pieces spread out over the globe? The earth's land mass was once concentrated in a single continent called Pangaea. Eventually plates broke apart, and they have been moving into their present day locations ever since.
Plates in the earth's crust squished together to make the Appalachian Mountains, then stretched apart to make the Atlantic Ocean Basin. Over time, pieces of the mountains (the Appalachians and Piedmont) washed out to the ocean and piled up as layers of sediment to make new land (the Coastal Plain). The Piedmont rocks are much harder than the new layers of sediments in the Coastal Plain. The letter A in I.M.P.A.C.T. represents evidence of two important tectonic events within this area (Appalachian Mountains and Atlantic Ocean basin). About 450 million years ago, North America and Africa were close neighbors, getting ever closer. The North American plate and the African plate collided, and the earth's crust reacted by folding and building up. Imagine what happens to a rug as it slides across a smooth floor; when it meets some sort of resistance it will start to wrinkle. In the case of continental crust, these "wrinkles" take the form of folds, faults, mountain building, and, in some cases, metamorphism. The collision between the North American and African plates was enough to build mountains as high as the 20,000+ foot Himalayas found today between the colliding plates of India and Asia. If we look at a relief map of the Washington, DC area today, we can see the wrinkled appearance of the Appalachian Mountains to the west, but they are nowhere near the height of 20,000 feet. The Appalachians rose higher and higher only as long as the rate of uplift was greater than the rate of erosion. Mountain building lasted for a few hundred million years, and then the plates reversed directions. The folded, crumpled layers of ancient sedimentary rock gradually wore away, leaving behind the exposed ridges of more resistant limestone and sandstone while softer shale layers eroded into valleys. This eroded sediment became the raw material from which the Coastal Plain was made. Approximately 200-250 million years ago, Pangaea began to break apart. The North American and African plates started to drift away from each other, and as they did, rift basins formed between them. Small tears in the earth's crust filled in with magma and left behind areas of igneous rock like ancient stretch marks. The largest rift between the plates filled in with water. This rift eventually became the Atlantic Ocean basin into which the eroded sediments from the rugged Appalachians began to collect, layer after layer. The Appalachians are the folded and wrinkled remnants of a taller, more rugged mountain chain whose foothills, or Piedmont, give way to the flat sedimentary Coastal Plain. Waterfalls and rapids can be found where rivers cross over the hard metamorphic and igneous rock of the Piedmont to the soft sedimentary rock of the Coastal Plain. At this contact, called the Fall Line, the Potomac River changes its appearance dramatically. Where the water cuts steeply through the more resistant rock of the Piedmont, it often follows faults and joints that run through the rock. The river's energy is focused in deep gorges and valleys, as visible at Great Falls. When the river meets the more easily eroded sediments of the flat Coastal Plain, however, it can meander and widen, as it does in the tidally influenced areas south of Georgetown. Since the river has an easier time moving and shaping the sediments of the Coastal Plain, it has worn them down to a lower elevation than the Piedmont. Ships could not navigate the falls and rapids of the Piedmont, so Washington, DC and many other early cities developed on the Costal Plain, right along the Fall Line.
The Chesapeake Bay is the largest estuary in the United States. Five freshwater rivers flow into a sheltered bay to mix with saltwater from the Atlantic Ocean. Research shows us that the Chesapeake Bay has a large meteorite to thank for its formation. A giant impact crater marks the mouth of the bay. It has taken us almost 450 million years to get through the first four letters of I.M.P.A.C.T., but it will only take a second to change everything with the C (Crater from the Chesapeake Bay meteorite). Approximately 35 million years ago, a meteorite about a mile in diameter crashed into the earth at what is presently the mouth of the Chesapeake Bay. Before the meteorite there was no bay, only several separate rivers cutting across the Coastal Plain, emptying into the Atlantic Ocean. Upon impact, the meteorite created a crater over 50 miles wide and thousands of feet deep, forever changing the courses of those rivers. Many of the ancient rivers took drastic bends and directional changes to converge at the site of the crater before continuing together out to the ocean. As rivers cut across the Coastal Plain at steeper inclines, deep canyons were easily carved into the soft sediments. Over time, these canyons became increasingly deeper as ocean levels dropped during the last Ice Age. Ocean levels later rose 600 feet when the glaciers melted, flooding the canyons. The result was the creation of Chesapeake Bay -- the largest estuary in the United States. It is a sheltered bay where the ocean's waters rise and fall to meet the fresh water of five major rivers (Susquehanna, Potomac, Rappahannock, York, and James). The Chesapeake Bay meteorite affected water below the earth's surface as well. Major underground water sources, or aquifers, were destroyed by the impact crater, leaving behind layers that now hold super saline water. Records of salty water in area wells go back as far as the Civil War when fresh water was needed for the Union troops at Forts Monroe. (This also happened during WWI and WWII). Maps of these "salty wells" show a peculiar ring pattern that has only recently been explained by geologists as the impact crater. Drill samples, rock fragments, and seismic mapping of the bay floor confirm the existence of the impact crater.
It took hundreds of millions of years for this area to look the way it does. The earth is constantly changing, even now, but it takes so long it's sometimes hard to tell. Finally, we come to the T in I.M.P.A.C.T. (Time). Geologists have their own timescale to break the earth's history into 4.6 billion years. By looking at the clues (minerals, fossils, and patterns) inside a rock as well as its relationship to the rocks around it, geologists see millions of years of stories. They observe how fast or slow mountains form or valleys erode today and assume that is how long it took millions of years ago too. When you add up all the stories, you end up with a lot of elapsed time. Sometimes it is difficult to imagine how events that took place millions of years ago can still impact our lives today. Yet, the Chesapeake Bay impact crater clearly affects the ground water sources and possibilities for development in that area. Also, the presence of sheltered, interconnected waterways influenced settlement, trade, and recreation in the Bay and along its contributing rivers. Flat open lands of the Coastal Plain with easy water access made ideal places for cities to grow, while other places were settled because of unique mineral deposits or trade routes. Forts and battlefields were often impacted by the occupation of strategic natural landmarks -- hills for look outs, or waterways for transport. Development will impact the history of an area as new people and ideas interact. Natural features, such as the Great Falls of the Potomac, were at first natural obstacles that inspired incredible feats of engineering such as the C and O Canal. Today the natural beauty of such sites inspires us in different ways.
It can be difficult to fully understand all the complexities of an area's geology, so make it simpler. The I.M.P.A.C.T. of geology on Washington, DC is easy to remember when you Imagine the Movement of Plates that created both the Appalachian Mountains and the Atlantic Ocean basin along with the Crater from the Chesapeake Bay meteorite that helped shape the Washington, DC area over Time. Geology manages to impact almost every aspect of the topography, development, and history of a region. The next time you hike a trail, or fly in an airplane, or walk down the street, look around and see if you can uncover the story of the earth in the everyday geology around you.
The geologic overview of the Washington, DC area lays the foundation for finding how geology influences the scenery, development, and history of this area. DC's geology plays an important role in why this location was chosen to be the site of the new capital city. Once chosen, DC's rapid development created irreversible impact- namely the increased sediment in the Potomac River. A dredging and reclamation project made the river once more navigatable, and at the same time extended the land area of the National Mall to the west and south with fill from the project. On this reclaimed land, memorials were erected to honor presidents and war veterans, each constructed with stones that not only provide the building blocks of each structure, but strengthen the themes and ideas of each memorial as well.
As a National Park site, the National Mall preserves and protects these great stone symbols. Because of all the building stones, a visit to the National Mall is like a geology field trip across the globe, including rocks from all 50 states!
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 transportation was too difficult to bring in stone from other parts of the country. As railroads expanded in the late 1800s, the choice of building material 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.
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.
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.
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.
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.
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.
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.
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.
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 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 approximately 500 million years ago. 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.
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.
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.
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.
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.
Sand and mud and gravel have been deposited over the last one hundred million years to create 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, DC buildings including the White House, Capitol, and National Portrait Gallery. The best place to see this sandstone up close is at the intersection of 15th Street and Constitution Avenue NW, where the Capitol Gateposts now stand.
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.
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.