A 60-Million-Year Recipe for Stone Spheres
Scattered along Koekohe Beach, the Moeraki Boulders appear as giant marbles left by a colossal player. These are septarian concretions, and they formed on the seafloor during the Paleocene Epoch, roughly 60 million years ago. The process began inside a layer of marine mud, which would later become mudstone. Each boulder started its existence around a central nucleus, a fragment of a fossil shell, a piece of bone, or other organic detritus.
Similar to how a pearl forms, minerals precipitated from the water and cemented the surrounding mud, silt, and clay. The primary cementing agent was calcite (calcium carbonate). The nearly perfect spherical shape of most boulders indicates that the calcite source diffused evenly from the central core. This process was extraordinarily slow. The largest boulders, some exceeding 2.2 meters in diameter, are estimated to have taken between 4 and 5.5 million years to form while tens of meters of mud accumulated on the seafloor above them. The main body of the boulders consists of this calcite-cemented mud, with a much harder outer rim containing 10 to 20 percent calcite.
Cracks, Crystals, and Coastal Reveal
Many of the boulders are broken open, showing a network of cracks and crystalline patterns. These cracks, known as septaria, formed after the concretion had hardened. Geological stress or shrinkage of the clay-rich core caused the boulders to fracture from the inside out.
These fractures did not remain empty. Over time, they were filled by minerals precipitating from groundwater. An early stage of filling deposited brown calcite, followed by a later stage of yellow calcite spar that often filled the remaining gaps. In some rare cases, a final, thin layer of dolomite and quartz was also deposited inside the cracks. Today, these boulders are visible because the softer mudstone cliff face that encased them for millions of years is steadily eroded by the persistent action of the waves. The boulders are not rolling onto the beach from elsewhere; they are being exhumed in place from the Paleocene-era Moeraki Formation mudstone.
Local Māori legend offers a different origin. The boulders, or Te Kaihinaki, are the petrified remains of eel baskets, calabashes (gourds), and kūmara (sweet potatoes) that washed ashore from the wreck of the legendary Āraiteuru canoe. This great canoe is said to have capsized near Shag Point (Matakaea), and its petrified hull is said to be the rocky shoals that extend out to sea from the point.