Contemporary Art
of
Ruth Killoran

 

Limestone

Limestone is a sedimentary rock abundant in New Zealand, it consists mainly of the bones and shells of tiny marine fossils made of lime (calcium carbonate). Rocks with more than 50% calcium carbonate are considered to be limestone.

Many different types of limestone exist, yet the mineral composition of all limestone is similar. Most of them are of marine origin, formed in ancient warm shallow seas directly and indirectly from the life processes of marine plant and animal organisms.

procheloniceras fakes

Most New Zealand limestone deposits formed in the Oligocene and Miocene periods, 5–37 million years ago. Around 20 to 30 million years ago, when the climate was warmer and much of New Zealand was submerged under shallow seas, conditions were ideal for limestone formation.

Not much sediment from land entered these coastal waters, and layers of shells and bones from billions of sea creatures accumulated on the sea floor. As these organisms died their calcareous remains, shells and skeletons accumulated in these sea beds building up vast deposits.
Accompanying this process, algae and other lime-secreting plants precipitated calcium carbonate in very fine clay to silt size particles which mixed with these fragments to form a calcareous ooze (mud). During these processes a wide variety of foreign materials ranging from clay minerals, quartz sand, iron oxides, and other minerals along with the remains of other plants and animals were being added to this sedimentary brew.

As the waters receded these deposits consolidated to form limestone rock masses. In some limestone's the partial or complete fossilized remains of these extinct organisms can be seen.

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These hardened into rocks, which were eventually uplifted and now form the country’s karst (weathered limestone) landscapes. They are a dramatic sight at Castle Hill on the road to Arthur’s Pass, and form the intriguing Pancake Rocks at Punakaiki on the West Coast. Other sites include the Waitomo caves in the King Country, and the rolling land around Ōamaru in North Otago, which is known as ‘white stone country’

Although there is a lot of limestone in New Zealand, much of it is too hard or fractured to be used for construction. The limestone around Ōamaru is used for building and sculpture. As it is relatively soft it can be cut in to blocks by huge circular blades, these can then be cut into smaller blocks that are easier to handle.

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When the stone is removed from the Quarry floor, it has a large moisture content, and when the milling of the stone is complete it will stay damp and soft for some time. It is then placed on timber pallets up to 1.5 tones in weight. These pallets are not covered when they leave the quarry, to allow air movement around the stone, and to allow the dispersion and evaporation of water. It is important to remember as the stone dries it becomes harder.

(Courtesy of ‘Explore Te Ara: The Encyclopedia of NZ’ and Parkside Quarry)

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