Limestone is a sedimentary rock (rock laid down as layers). It consists mainly of shells of tiny marine fossils made of lime (calcium carbonate, CaCO3). All limestones also contain some sand or mud.
Rocks with more than 50% calcium carbonate are classified as limestones. With less than 50%, they are called a calcareous sandstone or mudstone.
Limestone outcrops are scattered in patches along the length of New Zealand. They occur mainly east and west of the main mountain ranges – not high in the mountains.
There is a good reason for this. Most limestones are made of shell fragments and lime muds, originally deposited in shallow seas. They were compressed and cemented together after sediments (such as siltstones and sandstones) were deposited on top. Then tectonic forces folded, fractured and uplifted the limestone. Areas that were uplifted first were eroded first. Limestone that once extended across mountain tops has been worn away, but remnants often survive on areas that were not lifted as high (for example Castle Hill in Canterbury).
Sometimes limestone also contains magnesium carbonate. The double carbonate of calcium and magnesium is called dolomite – CaMg(CO3)2. In New Zealand, dolomite is only found in the small area of Mt Burnett, near Collingwood in Golden Bay.
When limestone is subjected to considerable pressure and heat, it recrystallises into marble. Fossils disappear in the process, but chemically the rock remains the same. Marbles occur in a discontinuous belt up to 7 kilometres wide for about 90 kilometres in north-west Nelson, extending from Mt Owen and Mt Arthur through to Tākaka Hill.
Limestone, dolomite and marble are collectively known as carbonate rocks. Carbonate rocks with more than 10% sand, silt or mud are considered impure. Karst develops best in pure limestones and marbles. In New Zealand most karst is found in rocks with less than 5% impurities.