How a Stone Grows Inside Sediment
A spherical boulder can begin as a patch of loose sediment, not as a ball of molten rock. Mineral-rich water moves through the sediment, and dissolved minerals gradually bind grains together into a harder mass. This process is called cementation: minerals act like a natural glue, turning the material around a small starting point into solid stone.
The concretion grows within the surrounding sediment. Its shape reflects how the cementing minerals spread through that material; where conditions allow growth outward in many directions, the result can be roughly round. The precise recipe varies, so not every concretion forms in the same way or becomes a perfect sphere.
Why the Boulders Appear on Beaches
The striking contrast comes later. Waves and weather wear away the softer sedimentary rock around a concretion faster than they wear down its more strongly cemented interior. As the surrounding material disappears, the hard mass is left exposed. Continued erosion can free it completely, while waves shift it along the shore.
That makes a beach look as if someone scattered stone balls across it. In reality, the boulders were hidden in the rock before the beach took shape; erosion did the revealing. Their smooth outlines are not evidence that they were rolled into spheres by surf. The durable stone began forming underground, and the shoreline gradually stripped away its softer surroundings.
