This monument was begun in 1848, but it was not finished until 1885; the
change in color about 150 feet up marks a change in the type of marble
used to face the monument. Although it is made of marble, its smooth,
straight shape and the massive blocks used in this monument have
minimized the effect of acid precipitation. Dissolution does occur in a
few areas, but the amount of stone material lost from dissolution is
insignificant compared to the mass of the stone.
[Illustration: The straight shape and massive stones in the
Washington Monument minimize the impact of acid precipitation to
this important landmark.]
Our tour ends here, but there are many more stone buildings and
monuments in Washington and in other cities that may also show the
effects of urban pollution and acid precipitation. However, as seen on
this tour, not all of the deterioration of stone buildings is caused
by acid rain. To protect our historic stone buildings and monuments,
we need to limit air pollution that contributes to acid rain
deterioration, and we need to develop effective maintenance and
preservation procedures that will not further harm the stone.