Giant cave systems have formed through the reaction of acids with carbonates the that make up limestone.
Caves are formed as a result of dissolution of limestone, through a type of chemical weathering known as carbonation. This happens when rainwater picks up carbon dioxide from the air and as it percolates through the soil, resulting to the formation of weak carbonic acid.
The weak carbonic acid formed then slowly dissolves out the limestone along the joints, bedding planes and fractures, some of which become enlarged enough to form caves.
Largest caves are formed at a place where the water flows onto the limestone from an adjacent impermeable Portishead formation and Avon Group mudstones.
