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.
Different minerals can be recognised within the septarian veins. The early filling is with a brown calcite, while the later filling is with a yellow calcite spar. In some instances, there is an even later filling of dolomite and quartz. The composition of the mineral that first fills the veins is similar to the mineral that cements the outer rims of concretions, suggesting that the cracks opened up at about the same time as the cement hardened to form the spheres.
A few ideas have been suggested for the cause of the septarian cracks including shrinkage of the core of the concretion by dehydration or compaction. However, it is difficult to envisage how core shrinkage would produce cracks in the outer parts of the spheres. To my mind, the cracks suggest that the outer parts of the sphere may have contracted as the cement was hardening and the material had become more brittle than plastic.
The local Maoris have an imaginative legend about the formation of the boulders. They say they are the remains of kumara (sweet potatoes), eel baskets and rounded fruit from the calabash vine that washed ashore after a large sailing canoe, the Arai-te-uru, was wrecked.
Interpreting the boulders within a larger geological framework, the Moeraki Formation has been classified as Palaeocene, due to the types of foraminiferans found in the muddy sediments. Called forams for short, these single-celled organisms secrete tiny skeletons with fascinating shapes, and these are used to correlate geological strata from one place to another. Based on the stratigraphic sequence of the area and the fossil ages assigned, muddy sea-floor conditions are interpreted to have persisted from the Cretaceous to the Eocene. The Palaeocene started about 65mya and these muddy conditions would have lasted for more than 20myrs, which is more than any of us can imagine. Calculations based on diffusion models for the formation of the concretions estimated that the larger 200cm boulders would have taken about 4myrs to grow.
Jocelyn Thornton, Field Guide to New Zealand Geology, Heinemann Reed, 1985.
Boles, J.R., Landis, C.A. and Dale, P., The Moeraki Boulders; anatomy of some septarian concretions, Journal of Sedimentary Research 55(3):398–406, May 1985.