When my daughters saw these rocks and asked me where they came from, I suggested that maybe aliens planted them there. They didn’t believe me. However, Maori legend has it that the boulders are the remains of eel baskets, calabashes, and kumera washed ashore from the wreck of a large sailing canoe (Āraiteuru) that was wrecked at nearby Shag Point (Matakaea).
Some 60 million years ago, the Moeraki boulders on the Otago coastline of New Zealand started forming on the ocean floor. Centuries of coastline erosion have revealed a spectacular view of these curiously large spherical boulders.
The largest boulders are as big as 3 metres in diameter and weigh several tons. It is estimated that they have taken 4 to 5.5 million years to grow. The boulders consist of mud, fine silt, and clay, and cemented by calcite. The degree of cementation varies from being relatively weak within the interior of a boulder to quite hard within its outside rim.
This next photo shows large cracks known as septaria formed in the boulders. 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. This particular large boulder has split over time revealing its inside.