Sponges are often preserved in flint. The flint comes from silica that originated in the tiny needles or ‘spicules’ that other sponges used to build their bodies. The needles were scattered and buried in the sediment, then later dissolved away by ground water percolating down through the chalk.
Later still, that silica could come out of solution, particularly where it found a silica hotspot in the form of another sponge. In this case, the entire sponge has been encased in silica with just some of the fine details of the structure now visible on the inside of this mould.
Find out more on the sponges page.