Why are limestone buildings damaged by acid rain?

Marble ad limstone are bases, and they are chemically reacting with the acidic properties of the rain.

This is because the sulphuric acid in the rain chemically reacts with the calcium compounds in the stones (limestone, sandstone, marble and granite) to create gypsum, which then flakes off. This is also commonly seen on old gravestones where the acid rain can cause the inscription to become completely illegible. Acid rain also causes an increased rate of oxidation for iron. Visibility is also reduced by sulphate and nitrate in the atmosphere.