1. As rainwater falls and passes through the air, it dissolves carbon dioxide.
  2. Carbon dioxide mixes with the rainwater to form a weak carbonic acid.
  3. Limestone in the ground contains calcium carbonate. When the weak carbonic acid in the rainwater falls onto the limestone, a chemical reaction takes place.  
  4. The calcium carbonate in the limestone begins to dissolve. This process is known as carbonation.
  5. Limestone is a permeable rock, which means that rainwater can easily pass through it. It has vertical cracks, which are known as joints, and horizontal bedding planes.