These unusually large and spherical boulders lie along a stretch of Koekohe Beach on the coast of New Zealand. Their origin has been the subject of numerous Maori legends but scientifically speaking they are the result of coastal erosion.
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 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 sea floor. 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, 2 metres (6.6 ft) in diameter, are estimated to have taken 4 to 5.5 million years to grow while 10 to 50 metres (33 to 164 ft) 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.