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.
Local Māori legends explained the boulders as the remains of eel baskets, calabashes, and kumara washed ashore from the wreck of Arai-te-uru, a large sailing canoe. This legend tells of the rocky shoals that extend seaward from Shag Point as being the petrified hull of this wreck and a nearby rocky promontory as being the body of the canoe's captain. In 1848 W.B.D. Mantell sketched the beach and its boulders, more numerous than now. The picture is now in the Alexander Turnbull Library in Wellington.The boulders were described in 1850 colonial reports and numerous popular articles since that time. In more recent times they have become a popular tourist attraction, often described and pictured in numerous web pages and tourist guides.
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 1.0 metres in diameter, the other two-thirds from 1.5 to 2.2 metres, 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.
Neither the spherical to subspherical shape or large size of the Moeraki Boulders is unique to them. Virtually identical spherical boulders, called "Koutu Boulders", are found on the beaches, in the cliffs, and beneath the surface inland of the shore of Hokianga Harbour, North Island, New Zealand, between Koutu and Kauwhare points. Like the Moeraki Boulders, the almost spherical Koutu Boulders are as large as 3 metres in diameter.
Similar boulder-size concretions, known as "Katiki Boulders", are found on the north-facing shore of Shag Point some 12 miles south of where the Moeraki Boulders are found. These concretions occur as both spherical cannonball concretions and flat, disk-shaped or oval concretions. Unlike the Moeraki boulders, some of these concretions contain the bones of mosasaurs and plesiosaurs.
Large spherical concretions, similar in size and shape to the Moeraki Boulders have been found elsewhere in the world. For example, large spherical concretions as large as 3 metres in diameter are along the Cannonball River within Morton and Sioux Counties, North Dakota. Large spherical concretions as much as 4 to 6 metres in diameter occur within sandstone outcrops of the Frontier Formation in northeast Utah and central Wyoming. Similar somewhat weathered and eroded giant spheroidal concretions, as large as 6 metres in diameter, are at Rock City in Ottawa County, Kansas. Smaller spherical concretions are found on the shore of Lake Huron near Kettle Point, Ontario, where they are known as "kettles".
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