Limestone is a rock composed mainly of lime (calcium carbonate – CaCO3), almost always formed from shelly material. Some limestones are very fine-grained, originally being a limey ooze on the sea floor, but in other rocks large shell fragments can be seen. Limestone is widespread in New Zealand, and formed mainly in late Eocene to Oligocene time, 25–40 million years ago, when much of the New Zealand region was submerged beneath the sea.
The hardness of limestone depends on how deeply it has been buried. Softer rocks such as Ōamaru stone have been compacted, but can still be carved and cut with a saw. Most other limestones have been more deeply buried. The calcium carbonate has crystallised in the pore spaces to form a hard, dense rock that can be polished. When limestone is deeply buried and heated it recrystallises so that the original shelly material cannot be recognised, and it is then called marble.
New Zealand’s most important building stone is a very pure, massive limestone, soft enough to be readily sawn and worked. It is composed mainly of microscopic bryozoa that accumulated as a submarine shellbank in the Ōamaru area. Because of its purity, it is mainly white. Small amounts of volcanic ash give the lower part a yellow tinge, and result in a poorer quality stone.
With pride in its heritage of neoclassical buildings, Ōamaru has adopted the name Whitestone City for promotional purposes, and holds an annual heritage week in November. Historian Erik Olssen regarded the main street of Ōamaru as a political statement: ‘Ōamaru’s leaders celebrated, in glossy white limestone, the triumph of the pioneers and the certainty of progress through capitalism.’ 1
Quarrying began about 1860, and Ōamaru stone was obtained from many different quarries inland and south of Ōamaru. Virtually all stone used for building since the 1940s has come from Taylors or Parkside quarries at Weston, and the latter still works the purest limestone. To extract the stone, it is cut into 2-tonne blocks by a chainsaw cutter, and broken out with a fork-lift loader. In an adjacent factory these blocks are cut into slabs by large circular saws.
The ease of dressing Ōamaru stone made it popular for buildings throughout New Zealand. It has been widely used for decorative work around windows and doors, as well as carved pillars and gargoyles. After several decades it tends to weather and flake. Many older buildings have needed cleaning and repair, but this can now be largely avoided if the stone is sealed.
Limestone called marble
Most limestones are harder than Ōamaru stone, and are much more difficult to work. They have, therefore, been mainly used for decorative facing stone. Although the term marble is often used for such rocks, the original shells are still visible, and they are technically limestone.
Whangārei marble is the commercial name for pale-coloured limestone quarried near Whangārei and widely used for facing stone. It takes a high polish, and has been used for decorative facing panels in several buildings. Similarly, Hanmer marble is the commercial name for a dense limestone with a pinkish-brown colour found in the Waiau valley, north Canterbury. The distinctive colour is due to the presence of small amounts of volcanic ash.
Different quarries sometimes give their own names to the rock they produce, even if it is similar to rock produced nearby. Tākaka marble has also been called Nelson marble and Caanan marble. All these names are from the same area of marble that geologists call Mt Arthur marble.
Tākaka marble is a true marble that occurs in the area around Tākaka, and has been worked from a number of quarries on Tākaka hill or in the nearby Holyoake valley since the early 1900s. Impurities vary from place to place, so each quarry produces distinctive coloured marble, ranging from white to pink or grey, and locally almost black.
The new Kairuru quarry, in the Holyoake valley, was the main producer in 2005 – mainly building panels and tiles. The rock is a coarse, crystalline marble with orange veins and irregular staining. The old Kairuru quarry, a little further down the hill, produced the grey marble used in Wellington’s Parliament Buildings.