*** THESE ARE ACTUALLY BOULDERS AT MOERAKI BEACH, NEW ZEALAND. *** The "Boulders" on the Moeraki beach are spherical concretions which formed within the surrounding mudstone millions of years ago when the area was covered by sea. The process by which concretions are formed is not well understood, but it is similar to the way a pearl forms around a particle in an oyster. The particle around which these "Boulders" formed might have been a small fossil shell, a bone fragment, or even a scrap of wood.
The "Boulders" are built up from a mineral called calcite. When they are exposed to the weathering effect of the sea they lose their outer layers, and this leaves hard veins of crystallized calcite standing out in relief - reminiscent of the flax-covered water containers of the Maori legend that explains how they come to be at the Moeraki beach. As the weathering progresses the veins control the break-up of the "Boulder" into small polygonal form.
A large concretion probably took about 4 million years to form. The Moeraki "Boulders" are not unique, but they are certainly remarkable for their large size of more than 2 metres (6.6 ft) in diameter, their highly spherical shape, and their concentration at the Moeraki beach. (Taken at South Island, New Zealand)
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