Scattered over Koekohe Beach on the South Island of New Zealand, dozens of huge spherical boulders look like the remains of a monster game of marbles. These were recently featured on the cover of Issue 22 of Deposits. The grey, stone balls are a fascinating tourist attraction, about 70km north of Dunedin, near Moeraki, a small town on the Otago Coast. Some boulders stand alone, but most sit in clusters, with the waves splashing over them at high tide. Many lie broken into segments on the sand.
The boulders are spectacular examples of concretions, which form when a mineral precipitates and cements the loose grains of sediment into solid rock. As you walk down the steep bluff to the beach, you can see other enormous boulders still embedded in the uncemented mudstone, but being exposed as the ocean waves erode the loose embankment. They eventually fall onto the beach.
The boulders come in two distinct sizes: the diameters of the smaller ones range from 46cm to 92mm, but the larger ones are 137cm to 200cm in size. The largest ones weigh almost 20 tons. Most are spherical, but a few are slightly squashed in a direction parallel to the bedding of the mudstone in which they formed.
Although fascinating, the boulders are by no means unique. In New Zealand, you can find similar ones on a beach just 12km south, and others on the North Island along the shore of Hokianga Harbour. In other parts of the world, similar boulders can be found, including in the cliffs along the Wessex Coast of England in a deposit called the Kimmeridge Clay; on the shore of Lake Huron near Kettle Point, Ontario; within sandstone outcrops in central Wyoming and northeast Utah; and at Rock City in Kansas, to mention but a few.
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The Moeraki Boulders formed within a thick deposit of mud, silt and clay called the Moeraki Formation when the grains of sediment were cemented by a mineral called calcite. The outside shell of each boulder is hard, with the calcite content being as high as 20% and even replacing the sediment to a certain extent. The insides of the boulders are not so well cemented and are eroded by the waves after they break apart. The spherical shape suggests that the mineral cement was precipitated through some sort of a mass diffusion process around a central point rather than by groundwater flow. Perhaps the process was similar to the precipitation reactions that form Liesegang rings in fluid-saturated sediment.
Multiple cracks, filled with a calcite deposit, radiate outward from the cores of the boulders, becoming thinner toward their tips. They are called ‘septaria’ (Latin: septum = partition) because they separate the boulders into a number of distinct segments. Because of these cracks, the surface of the boulder appears to be composed of segments like a soccer ball. And it is because of these cracks that the boulders are described as septarian concretions.