The Moeraki Boulders are stone spheres up to two metres across, lying loose on a beach in Otago, in the south of New Zealand. Nothing carved them. Each one grew, from the inside out, around a single speck on the sea floor. Lime gathered on a shell fragment or a grain of sand in soft mud sixty million years ago. Over the next four or five million years it built up into a ball, hardening while the mud around it stayed soft. So when the cliff wears back, the balls come out whole. At the rear of the beach the low mudstone bank is bulging with the next ones, half-exposed spheres locked in the rock that made them, one or two already cracked open where they sit.
At low water the whole field opens. You can walk it from the tight group near the path out to the lone spheres on the wet sand, where the biggest come up to your chest and weigh several tonnes. The beach faces east, so the sun comes up out of the Pacific and turns the wet grey shells copper. Then the tide comes back, running around each sphere and climbing it, until the smaller ones are dark domes in the foam. Close up, the surface of a big one resolves under your hand into a raised net of veins, like the seams on a turtle's shell, where the filled cracks have outlasted the softer stone between them. There are initials scratched into that surface as well, and the beach is a scientific reserve now for the same reason. When Walter Mantell sketched this shore in 1848 he drew far more spheres than stand here today, and noted the small ones were already going to garden paths and villa grottoes. A split boulder shows you what the whole business was. The cracks inside were not filled in one go: brown calcite first, then a late yellow spar, then rare dolomite and quartz. Some of the inner cavities never filled at all, so a broken sphere can open like a geode, lined with sharp crystals.