Stop 4: A Watery Past

To find the symbolic center of the National Mall draw an imaginary line south of the White House and west of the Capitol. The location where the two lines intersect was the proposed site of a monument to George Washington in the city that bears his name.

Instead, if you follow those lines today, you will find a small marker called the Jefferson Pier Stone. The Washington Monument could not be built in the symbolic center of the National Mall because a structure that large would have toppled into the Potomac River. At that time, the shoreline reached the Pier Stone. Nearby stands an old lock keeper’s house, leftover from the days of the Washington City Canal, which is now paved over by Constitution Avenue.

Today, you cannot see the Potomac River shoreline from the Pier Stone. The area to the west and south of that point was filled in with sediment that came from the bottom of the river during the late 1800s reclamation project. Now, you can travel from the Pier Stone to the Lincoln Memorial by walking on the former river bottom without getting your feet wet!


Jefferson Pier Stone

On December 18th, 1804, a simple granite obelisk was erected at the intersection of lines from the front doors of the Executive Mansion and the Capitol Building, as part of a meridian system used to align city streets and development. As President, Thomas Jefferson wished for the United States to become scientifically as well as politically independent from Europe, so he wished for the new national capital itself to be a new “first meridian.”

The original marker aided surveyors and later served as a benchmark during construction of the Washington Monument. As Tiber Creek was transformed into the Washington City Canal, the marker became known as the “Jefferson Pier” because barges navigating the Potomac River routinely used the original marker as an anchoring post. The entire Mall area west of the Pier Stone was once under water!

The original marker disappeared in 1874, but a replacement marker was erected December 21, 1889. This simple obelisk is about 100 yards on a diagonal from the northwest corner of the Washington Monument. In addition to offering a glimpse into the dramatic changes the National Mall has undergone, this spot offers marvelous views of the surrounding monuments. When viewed at night from this vantage point, flashbulbs going off in the chamber of the Lincoln memorial look like fireflies on a warm summer night.

Washington City Canal

Several small streams in Washington, D.C. were turned into a canal system that connected the Washington Waterfront, the Capitol, the White House and other areas downtown with the C&O Canal’s first lock in Georgetown. A lock keeper was needed along the Washington City Canal to help boats navigate in the mid-to-late 1800s. The canal has since been paved over, but water still flows under Constitution Avenue. Today the Old Lock Keeper’s House can be found at 17th Street and Constitution Avenue. Just a few streets away are the Bulfinch Gatehouses and Gateposts that originally stood on the Capitol Grounds.

If you compare the rock used in the construction of the Gateposts and the Lock House, you can see why canals and locks were needed. The Gateposts are made from a Coastal Plain rock called Aquia Creek sandstone. The Lock House is made from crystalline Piedmont rocks called the Sykesville Formation. The soft sandstone used in the Gateposts is wearing away much faster than the hard metamorphic rock of the Lock House, just like the soft Coastal Plain sediments wear away much faster than the hard Piedmont rocks.

Washington, D.C. sits on the Fall Line where those two types of rocks are found side by side. Since the Potomac River becomes rocky and steep when it flows through the Piedmont, canals and locks were needed along the Potomac River to move ships upstream of Georgetown where the Costal Plain ends.

Continue Your Tour of the National Mall

Stop 1: The Geology of the Washington D.C. Area

Stop 2: The History of Washington D.C.

Stop 3: Finding D.C.’s Foundation

Stop 4: A Watery Past

Stop 5: GeoStory of the Lincoln Memorial

Stop 6: Remembering War

Stop 7: Stories in Stone at the Franklin Delano Roosevelt Memorial

Stop 8: Thomas Jefferson Memorial - A Place of Controversy

Stop 9: Washington Monument - The Nation’s Most Unique Rock Collection

Stop 10: Who Cares for the National Mall







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