New Zealand Has a Beach Scattered With Giant Stone Marbles

Picture a wide, pale beach on the east coast of New Zealand's South Island, with gentle waves rolling in and gulls wheeling overhead. Now scatter it with enormous grey balls of stone, some knee-high, some taller than a grown-up, sitting in the shallows and on the sand as if a family of giants had finished a game of marbles and wandered off for lunch. This is Koekohe Beach, between the villages of Moeraki and Hampden, and the stones are the Moeraki Boulders.

They really are startlingly round. About a third of them are between half a metre and a metre across, and the other two thirds measure from about one and a half to a little over two metres. Visitors climb on them, lean against them, peer into the ones that have split open, and take the same photograph at sunrise that everybody else has taken, which is entirely understandable.

The surprising part is that nobody carved them and nothing rolled them smooth. The boulders were not shaped by waves bashing them about. They grew. They are concretions: lumps that formed inside soft mud on the ancient sea floor, in the Paleocene epoch, roughly 66 to 56 million years ago, when this piece of New Zealand was still under the sea.

A concretion usually starts around something small, such as a shell or a piece of wood buried in the mud. Minerals carried by the water between the grains settle around that starting point as calcite, the same mineral that makes up chalk and limestone, and glue the mud, silt and clay together. Layer by layer the ball grows outwards in every direction at once, which is why it ends up round. The largest Moeraki Boulders, about two metres across, are thought to have taken between four and five and a half million years to reach their size. Patience, it turns out, is the secret ingredient.

Many of the boulders are crossed by cracks, called septaria, that spread outward from the middle and were later filled with calcite crystals. On the weathered surface they show up as a network of lines, a little like the seams on a well-loved leather football. Local Maori tradition tells a different and lovely story: the boulders are the cargo of the great canoe Araiteuru, wrecked on this coast, and the pattern on them is the canoe's fishing nets.

So how did buried stones end up on a beach? Geologists who studied them worked out that the boulders formed after the mud around them had been buried about 500 metres deep, at temperatures of around 25 to 35 degrees Celsius. Over a very long time the layers above were worn away. Today the soft, dark mudstone cliffs behind the beach are still being nibbled by the sea, and as the mud washes out, the far harder boulders are freed and left sitting on the sand.

The cracks tell part of that story too. The boulders formed deep underground, where the pressure was high, and as erosion slowly brought them nearer the surface they cracked while still inside the mudstone, before groundwater filled the gaps with fresh mineral. Some boulders are still hidden in the cliff, waiting their turn, so the beach is a gallery that very slowly rearranges itself. The area is protected as a scientific reserve, which means the marbles stay exactly where the giants left them.