thumb|upright=1.15|Boulders of varying sphericity broaching the [[Otago coast]]
thumb|The boulders at sunrise
The Moeraki Boulders (officially Moeraki Boulders / Kaihinaki) are unusually large 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. These boulders are grey-coloured septarian concretions, which have been exhumed from the mudstone and bedrock enclosing them and concentrated on the beach by coastal erosion. Especially in recent years, the boulders have been a popular tourist attraction.
Description
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 in diameter, the other two-thirds from . Most are spherical or almost spherical, but a small proportion are slightly elongated parallel to the bedding plane of the mudstone that once enclosed them. Similar septarian concretions have been found in the Kimmeridge Clay and Oxford Clay of England, and at many other locations worldwide.
Composition
thumb|A cracked boulder displaying its hollow interior
Detailed analysis of the fine-grained rock using optical mineralogy, X-ray crystallography, and electron microprobe has determined that the boulders consist of mud, fine silt and clay, cemented by calcite. The degree of cementation varies from being relatively weak in the interior of a boulder to quite hard at 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. Similar septarian concretions have been found in the Kimmeridge Clay and Oxford Clay of England, and at many other locations worldwide.
Origin
The Moeraki Boulders are concretions created by the cementation of the Paleocene mudstone of the Moeraki Formation, from which they have been exhumed by coastal erosion. The main body of the boulders started forming in what was then marine mud, near the surface of the Paleocene seafloor. This is demonstrated by studies of their composition; specifically the magnesium and iron content, and stable isotopes of oxygen and carbon. Their spherical shape indicates that the source of calcium was mass diffusion, as opposed to fluid flow. The larger boulders, in diameter, are estimated to have taken 4 to 5.5 million years to grow while 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 sea level drop allowed fresh groundwater to flow through the mudstone enclosing them.
Documentation and oral tradition
thumb|[[Albert Percy Godber portrait among the boulders (circa 1925)]]
Local Māori legends explained the boulders as the remains of eel baskets, calabashes, and kūmara washed ashore from the wreck of Āraiteuru, 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. Their reticulated patterning on the boulders, according to this legend, are the remains of the canoe's fishing nets.