Most of these boulders are 5-7 feet, while only a third of them are smaller. They are what are known as concretions which are hard, compact mass of sedimentary rock that form around a nucleus. The nucleus can be anything from a fossil to shrapnel from WWII.
Concretions form within layers of sedimentary strata that have already been deposited. They usually form early in the burial history of the sediment, before the rest of the sediment is hardened into rock.
The main body of the Moeraki Boulders started forming in what was then marine mud, near the surface of the Paleocene sea floor. The larger boulders, 6.6 feet in diameter, are estimated to have taken 4 to 5.5 million years to grow while 33 to 164 feet of marine mud accumulated on the seafloor above them.
