How flint was formed

In the late Jurassic period, some 155 millions of years ago, a shallow warm sea, which was part of the prehistoric Ocean of Thetis, covered the present-day territory of Poland. For many thousands of years lime deposits accumulated in a bay which existed where we now find ourselves. Marine crustaceans – crabs and shrimp – thrived in this environment, tunneling in the lime silt extensive networks of caves and passages which served them as havens against predators and sea currents. It was there that they built their nests and laid their eggs, and collected inside these tunnels organic remains, mainly plants. These decaying substances nourished colonies of bacteria which in turn constituted food for the larvae of the crustaceans.

The sea water in the bay had a high alkaline pH and contained large quantities of dissolved silica (SiO2). Decaying plants changed the pH of water in the tunnels. The drop to a more acid pH caused supersaturation, resulting in silica being precipitated in the form of a gel which gradually filled the crustaceans’ holes. The process could have continued even after all of the empty spaces were filled completely, thus giving rise to concentrations of flint forming concretions of fantastic shape. With passing time water disappeared from the silica gel and changed into a hard rock. This is flint.

Flint formation is reconstructed thus by Grzegorz Pieńkowski and Jacek Gutowski from the State Geological Institute. It is only one of a number of theories on this subject and studies of the origins of this beautiful rock are continuing.