WASHINGTON — The Washington Monument, the iconic marble tribute to the first U.S. president that strikes into the District of Columbia skyline, is sinking.
There’s no need to book a vacation to the capital to catch it before it slips away: The obelisk — which is 555 feet, 5 inches tall — has subsided only 2 inches since it was finished in 1884, according to new data from the National Geodetic Survey.
Even at that rate, though, the subsistence eventually could spell problems for the monument, especially after it was rocked by a magnitude-5.8 earthquake in August. Preliminary data collected Wednesday showed that the monument has sunk 2 millimeters since the last survey was done in 2009.
Dave Doyle, the government’s chief geodetic surveyor, is trying to determine how much of the sinking is a natural result of building an 81,120-ton stone pillar on reclaimed land, and how much was caused by last summer’s quake.
“People see the Washington Monument sitting on a nice little hill. They think that was always there, and it wasn’t; much of it was swampy,” he said.
In fact, the entire western end of the National Mall is built on former marshland, meaning the Lincoln and Jefferson memorials are sinking at about the same rate as the monument, according to Doyle’s measurements.
The Capitol and the White House are on firmer ground, and Doyle said there is no evidence they are sinking.
There is a slight chance that the Washington Monument is leaning to one side, too, in the manner of Pisa’s famous tower, but Doyle said more information is needed to check. From the ground, Doyle can detect changes of less than a millimeter, but later in the year, his team will take measurements from the top of the monument, which will give even more accurate data.
The National Park Service will have to determine what measures are necessary to stop or slow the downward movement of some of the nation’s most famous structures. Spokeswoman Carol Johnson said the service is still waiting for the final survey results.
“At this point we don’t have any reason to believe there’s anything going on that needs attention,” she said.
The interior of the Washington Monument remains closed after being damaged in the earthquake last summer, and is not expected to reopen until the winter of 2013.
While geologists study earthquakes and the structure of the Earth, geodesy is the science of determining its size and shape. It was practiced by the Ancient Greeks, who made a fairly accurate approximation of our planet’s circumference.
Doyle, a 40-year veteran of the National Geodetic Survey, today uses sophisticated electronic tools in his work. His predecessors at the survey did not have those, but Doyle said he is confident of the accuracy of the measurements taken in the 19th century, shortly after the monument was finished.
“We get virtually the same kinds of results that we got from instruments that go back over 100 years. They were just bigger and harder to use,” he said.
As for the monument’s long-term prospects, Doyle’s not too worried. “It’s real stable,” he said.