Moeraki Boulders Koekohe Beach, New Zealand

The Moeraki Boulders are unusually large and spherical boulders lying along a stretch of Koekohe Beach on the wave cut Otago coast of New Zealand between Moeraki and Hampden. They occur scattered either as isolated or clusters of boulders within a stretch of beach where they have been protected in a scientific reserve. The erosion by wave action of mudstone, comprising local bedrock and landslides, frequently exposes embedded isolated boulders.

These boulders are grey-colored septarian concretions, which have been exhumed from the mudstone enclosing them and concentrated on the beach by coastal erosion. The most striking aspect of the boulders is their unusually large size and spherical shape, with a distinct bimodal size distribution. Approximately one-third of the boulders range in size from about 0.5 to 1 meters in diameter, the other two-thirds from 1.5 to 2.2 meters, mostly spherical or almost spherical. A small proportion of them are not spherical; being slightly elongated parallel to the bedding of the mudstone that once enclosed them. The larger boulders, 2 meters in diameter, are estimated to have taken 4 to 5.5 million years to grow while 10 to 50 meters of marine mud accumulated on the seafloor above them. After the concretions formed, large cracks known as septaria formed in them. Brown calcite, yellow calcite, and small amounts of dolomite and quartz progressively filled these cracks when a drop in sea level allowed fresh groundwater to flow through the mudstone enclosing them.

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