Malta is composed mainly of various types of limestone. The Upper and Lower Coralline Limestones are used extensively for building. The Globigerina Limestone tends to be quite soft and can be cut with a saw, but as it is exposed to the air it hardens and can be used as a building stone. This is composed of microfossils, a foraminifera to be precise, which were planktonic and fell to the bottom when dead, to accumulate into fine grained limestone. This weathers to a sandy yellow colour similar to Bath stone.
Continental hydrogeology has ceased to exist in Malta and today only Għar Dalam and Għar Hasan exhibit the phreatic features of caves formed beneath the water table. Malta now has a typical island hydrology. Most of the rain, which falls in the winter period, October to December, escapes to the sea. Any water that is absorbed by the rock either emerges as springs at sea level or is stored in wells and cisterns. The water deficit is compensated for by large desalination plants.