Moeraki Boulders

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.

[citation needed] 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.

[1][4] 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.

Rare Moeraki Boulders have a very thin innermost (latest stage) layer of dolomite and quartz covering the yellow calcite spar.

[1][3][4] The composition of the Moeraki Boulders and the septaria that they contain are typical of, often virtually identical to, septarian concretions that have been found in exposures of sedimentary rocks in New Zealand and elsewhere.

This is demonstrated by studies of their composition; specifically the magnesium and iron content, and stable isotopes of oxygen and carbon.

[1][3][4][7] 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.