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.
http://en.wikipedia.org/wiki/Moeraki_Boulders#OriginI had to look them up to see if they were concretions. When I was a kid, we used to find marble sized (about 1/2" to 3/4" in diameter) concretions in the tailings from the phosphate mining around Bartow, Florida. A lot of the people in the area called them "Indian marbles" but my dad, a mining engineer explained the geology of how they formed. I never heard of a concretion any where near the size of those in New Zealand!
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