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.As determined by detailed analysis of the fine-grained rock using optical mineralogy, X-ray crystallography, and electron microprobe, the boulders consist of mud, fine silt and clay, cemented by calcite. The degree of cementation varies from being relatively weak within the interior of a boulder to quite hard within its outside rim. The outside rims of the larger boulders consist of as much as 10 to 20% calcite, because the calcite not only tightly cements the silt and clay but has also replaced it to a significant degree.The rock comprising the bulk of a boulder is riddled with large cracks called "septaria" that radiate outward from a hollow core lined with scalenohedral calcite crystals. The process or processes that created septaria within Moeraki Boulders, and in other septarian concretions, remain an unresolved matter for which a number of possible explanations have been proposed. These cracks radiate and thin outward from the centre of the typical boulder and are typically filled with an outer layer of brown calcite and an inner layer of yellow calcite spar, which often, but not always, completely fills the cracks. Rare Moeraki Boulders have a very thin innermost layer of dolomite and quartz covering the yellow calcite spar.
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