The Washington Monument is an obelisk on the National Mall in Washington, D.C., Constructed to commemorate George Washington, once commander-in-chief of the Continental Army (1775--1784) in the American Revolutionary War and the first President of the United States (1789--1797).
Located almost due east of the Reflecting Pool and the Lincoln Memorial, the monument, made of marble, granite, and bluestone gneiss, is both the world's tallest predominantly stone structure and the world's tallest obelisk, standing 554 ft 7 11?32 inches (169.046 m) tall according to the U.S. National Geodetic Survey (measured 2013--14) or 555 ft 5 1?8 inches (169.294 m) tall according to the National Park Service (measured 1884). It's the tallest monumental column in the world if all are measured above their pedestrian entrances. Overtaking the Cologne Cathedral, it was the tallest structure in the world between 1884 and 1889, and it had been overtaken by the Eiffel Tower in Paris.
Construction of the monument began in 1848 and was stopped for a period of 23 years, from 1854 to 1877 because of a lack of capital, a battle for control within the Washington National Monument Society, and the American Civil War. Although the stone structure was completed in 1884, inner ironwork, the knoll, and installation of memorial stones weren't completed until 1888. A difference in shading of the marble, visible approximately 150 ft (46 m) or 27% up, shows where construction was halted and later resumed with marble from a different source. The original design was by Robert Mills (1781--1855) of South Carolina, but he did not include his proposed colonnade because of a lack of funds, proceeding only with a bare obelisk. The cornerstone was laid on July 4, 1848; the first stone was set atop the unfinished stump on August 7, 1880; the capstone was set on December 6, 1884; the completed monument was dedicated on February 21, 1885; and officially opened October 9, 1888.
The Washington Monument is a hollow Egyptian style stone obelisk with a 500-foot (152.4 m) tall column surmounted by a 55-foot (16.8 m) tall pyramidion. Its walls are 15 feet (4.6 m) thick at its base and 1 1?2 ft (0.46 m) thick at their top. The marble pyramidion has thin walls only 7 inches (18 cm) thick supported by six arches, two between opposite walls which cross at the center of the pyramidion and four smaller corner arches.
The cover of the pyramidion is a large marble capstone with a little aluminum pyramid at its apex with inscriptions on all four sides. The lowest 150 ft (45.7 m) of the walls, constructed during the initial phase 1848--1854, are composed of a heap of bluestone gneiss rubble stones (not completed stones) held together by a large amount of mortar using a facade of semi-finished marble stones about 1 1?4 ft (0.4 m) thick. The upper 350 ft (106.7 m) of the walls, constructed during the next stage 1880--1884, are composed of finished marble surface stones, half of which project to the walls, partially backed by completed granite stones.The interior is occupied by iron stairs that spiral up the walls, with an elevator at the center, each supported by four iron columns, which do not encourage the stone structure. The stairs contain fifty sections, most on the north and south walls, with many long landings extending between them across the east and west walls. These landings allowed many inscribed memorial stones of different materials and sizes to be easily viewed while the stairs were available (until 1976), plus one memorial stone between stairs which is difficult to view. The pyramidion has eight observation windows, two per side, and eight red aircraft warning lights, 2 per side. Two aluminum lightning rods connected through the elevator service columns to ground water protect the monument. The monument's present foundation is 37 feet (11.3 m) thick, consisting of half of its original bluestone gneiss rubble encased in concrete. At the northeast corner of the foundation, 21 ft (6.4 m) below ground, is the marble cornerstone, such as a zinc case full of memorabilia. Fifty American flags fly on a huge circle of sticks based on the monument. In 2001, a temporary screening facility was added to the entry to stop a terrorist attack. An earthquake in 2011 slightly damaged the monument, and it had been closed until 2014. It was closed again for elevator system repairs, security upgrades, and mitigation of soil contamination from August 2016 to September 2019.