How flint forms
Banded (or striped) flints are something of a mystery as no-one is quite sure how they form. The flint itself originated from the spines of sponges that became buried in the ooze at the bottom of the chalk sea. Later, ground water percolating through the chalk dissolved the spines and carried the silica in solution down through the rock. Holes left by burrowing animals and sponges were filled with this water and the silica crystalized out to form flint. That is why flints are typically round in section and branching as they form inside burrows of that shape, or they contain sponges.
Find out more on the rocks and pseudofossils page.