Flints are made from silica that originally derived from sponges living in the chalk sea. The sponges are made up of millions of minute needles of silica locked together to form this simple animal. When the sponges died, the needles were scattered on the sea bed and buried in the sediment.
Later, ground water dissolved the silica and carried it in solution down through the rock layers. That silica-rich water gathered in cavities and spaces such as hollow burrows and other sponge bodies where the silica recrystallized, forming flint. That’s why flints are often long, thin and branching, where they filled burrows, or flask-shaped (like this specimen) where they formed around a sponge.
Find out more on the sponges page.