The Washington Monument is the tallest structure in DC. No other buildings are allowed (by law) to be taller than this structure.
If you closely, the Washington Monument has 2 colours. The reason is that the excavation for the foundation of the Washington Monument began in the spring of 1848 and construction continued until 1854, when donations ran out. After much delay, construction resumed in 1879. The stones used to proceed were taken from a different quarry and have a slightly different color than the bottom third of the obelisk. In four years, it was finally completed on December 6, 1884.
The reflecting pool: