The architecture of the nation’s capital reveals a secret geologic history—take a walking tour to spot the interesting fossils and minerals in the stones used to build the halls of power.

Washington, D. C., is much more than the seat of federal power. The city is the home of funky go-go music, the spiffed up hot dog known as a half-smoke, and striking architecture that hides geological gems for any curious Earth scientist.

In between Fall Meeting sessions, you can wander the National Mall to visit museums of our nation’s history (the most direct route from the convention center to the Mall is about 1.8 kilometers, or 1.1 miles, southward along 9th Street NW). But if you keep your eyes open, you can also take a tour of the “accidental museum” of fossils that are hiding in plain sight in buildings all around the city.

Here we’ve planned out a 4-kilometer (2.5-mile) walking tour from west to east along the National Mall (to save your feet for trekking through the convention center, you might want to rent a Bikeshare bike). You can even use our handy AGU Fall Meeting Google Map, where we’ve included a map layer with these tour stops. At the end of your tour, you’ll have a chance to carry your science past the walls of the convention center and into the halls of Congress.

But all along the way, you’ll have a chance to see how many earthly treasures aren’t preserved in museums at all. Like any other geologic feature, their beauty and secret histories are hiding in plain sight—right here in our urban landscape.

Stop 1: Vietnam Veterans Memorial

Our tour begins on the National Mall just south of Constitution Avenue NW, between 21st and 22nd Streets (about 3.5 kilometers, or 2.2 miles, southwest of the convention center), a short distance northeast of the white marble Lincoln Memorial. Here you’ll find the Vietnam Veterans Memorial, a tribute to soldiers killed and missing in action between 1955 and 1975.

Maya Lin’s moving design is constructed from a highly polished, ultramafic gabbro. The stone was imported from Bengaluru, chosen for its intense, somber color. Lore has it that there was a closer source for dark gabbro in Canada, but the Vietnam Veterans Memorial Fund chose to forgo that source.

The reason? Designers didn’t want to use rock from a country that accepted American conscientious objectors.

Stop 2: Lockkeeper’s House

Walk eastward along the Reflecting Pool past the World War II Memorial, then turn north toward the intersection of Constitution Avenue and 17th Street to see the Lockkeeper’s House, which used to mark the end of the C&O Canal system, which was used between 1830 and 1924 to transport many of the interesting stones you can now see in buildings all over the city.

The house is built with a variety of schists and gneisses known as Potomac bluestone, the official rock of D. C. Get up close to search for tiny garnets and deformed quartz pebbles.

Stop 3: Sykesville Formation

Walk out the front door of the Lockkeeper’s House, and you will see a small boulder in the grass, marked with a plaque describing the extent of the C&O Canal. This is a piece of the local Sykesville Formation, metamorphosed greywacke with big pebbles and cobbles.

The rocks are a mélange related to subduction that was capped by the first of the mountain-building events that formed the Appalachians. Take a side tour through Rock Creek Park (6.6 kilometers north) or Theodore Roosevelt Island (1.7 kilometers west) to see where else this formation appears.

Stop 4: Capitol Gatehouses

Diagonally across the intersection of Constitution Avenue and 17th Street NW is one of two Capitol gatehouses. The other is at 15th and Constitution, which still features the tall rectangular gateposts that secured fencing to keep grazing animals away from federal buildings. All of these structures are built from local Aquia Creek sandstone, the same stone used to build the White House.

The Capitol gatehouses and gateposts haven’t enjoyed the same constant upkeep as the White House, and you can see how they’ve held up to weathering—terribly! Look closer for delicate cross-bedding and rusty halos around oxidizing pebbles.

Stop 5: Second Division Memorial

Looking north from the gatehouse toward the White House Ellipse, you’ll see a pink stone monument, featuring a sculpture of a hand holding a flaming golden sword. This monument honors soldiers of the Second Infantry Division who fought in World War I, but it has a history of its own: The base is 3.5-billion-year-old Morton gneiss from Minnesota. Get closer to spot flowing bands of mafic minerals and pink feldspar.

Stop 6: Washington Monument

Turn to face southeast and look up to see this D. C. landmark. Note the color transition about one third of the way up the obelisk.

The bottom portion was constructed using Texas marble (from the town of Texas, Md.), but funding ran out around 1854, and the outbreak of the Civil War delayed construction. When it resumed, the original marble was no longer available, so the monument was completed with Cockeysville marble, also from Maryland.

Like an unconformity, the transition from dark to light stone is a kind of geologic boundary: a major event in human history preserved in the rock record.

Stop 7: Smithsonian Castle

Looking farther southeast, you’ll see the Norman-style architecture and red towers of the Smithsonian Castle. The headquarters of the Smithsonian Institution is composed of Seneca red sandstone, quarried roughly 32 kilometers (20 miles) north of D. C. and brought here on the C&O Canal. The rich color is a product of oxidation.

If you want to see more of the Seneca red sandstone, look for it in the stately brownstones around Dupont Circle (about 4 kilometers to the northwest). If you’re lucky enough to get a tour of the Capitol Building, you can see brightly polished samples in the floor of the Rotunda.

Stop 8: National Gallery of Art (West Wing)

Walk east along the Mall to 7th Street NW to reach the National Gallery of Art. Enter the West Wing from the Mall side (which faces Madison Drive) and head left toward the 13th–16th century Italian paintings.

The border of these rooms is made of Tennessee limestone, featuring nautiloids and bryozoans. Keep your eyes sharp as you continue; the floor is covered with fossil masterpieces.

Stop 9: National Museum of the American Indian

Walk southeast to 4th Street and Jefferson Drive to reach this relatively new addition to the city’s collection of museums. The museum strikes a unique profile next to the brutalist and neoclassical architecture of the other museums. On a sunny day, its buff facade catches the light like the cliffs of the American Southwest.

Look closely to discover trace fossils and fossilized worm burrows in the Kasota limestone of the lower panels. And if you’re hungry, this is where to stop for lunch: The cafeteria serves the cuisine of indigenous people from every region of the United States.

Stop 10: Capitol Reflecting Pool

Continue heading toward the Capitol and pause to admire the edge of the Reflecting Pool. Like many of the federal buildings in the neighborhood, it’s constructed of fossiliferous Indiana limestone. Constant weathering reveals tiny shells and exoskeletons in beautiful high relief.

Stop 11: Rayburn House Office Building

While you’re in D. C. to share your science with other AGU members, why not share your expertise with your congressional representative? A short walk eastward along Independence Avenue will bring you to the Rayburn Building, where many of the members of the House of Representatives have their offices.

Most offices are happy to meet with constituents. Afterward, pop into any of the Rayburn bathrooms to take a look at the gray limestone lining the walls and stall dividers to see stylolites, crinoids, and a variety of trace fossils.

The Geoscapes of Buildings

These are just a few places where you can see the evidence of geologic history preserved on a human scale in Washington, D. C. You can also look more closely for fossils in architecture around the city, spot other building stones at popular sites, or stray farther afield on local hikes to see D. C.’s building stones in situ (thanks to Callan Bentley and Ken Rasmussen for sharing their expertise).

From human wars to fossil remnants from ancient seas, the city’s buildings and monuments hold the hidden echoes of Earth’s history. What will you discover?

—Lily Strelich ([email protected]; @lilystrelich), Freelance Writer