The Seaford Chalk Formation

Found on the beach: a lump of wave-worn chalk cracked open to reveal the dark, honeycombed core of a Cretaceous burrow network. The white outer shell is soft Upper Cretaceous chalk, the familiar coccolith-rich limestone that forms the cliffs east of Brighton. Inside lies its harder counterpart, flint, formed when silica gel precipitated around voids in the ancient seabed. Over time the chalk eroded faster than the flint, leaving the interior exposed like a miniature cavern.

The tubes and chambers belong to Thalassinoides, the dominant burrow system of the Seaford Chalk Formation. Created by small crustaceans on a warm Cretaceous sea floor some 85 million years ago, Thalassinoides form semi-ordered meshes of uniform cylindrical tunnels. In life these burrows aerated the soft carbonate mud; in death they provided ready-made moulds for the silica that later hardened into flint. The network here is unusually clear: a continuous dark core threaded with branching passages, visible from several angles where the chalk shell has been scoured away.

Nothing in the piece is modern. The perforations are not the work of piddocks or contemporary worms but the preserved architecture of Cretaceous seabed life, frozen in flint and released again by the waves. What looks at first like an odd skull-shaped pebble is in fact a three-dimensional cross-section through an ancient ecosystem - a Brighton Beach fossil in miniature, shaped by crustaceans, lithification and the long slow abrasion of the Channel.

Sources: The British Geological Survey; Wikipedia; Earthwise