HOLLOW IRONSTONE CONCRETION - Bedfordshire

In unconsolidated sedimentary rocks iron is often dissolved in the groundwater and can be redeposited as hard, insoluble layers of iron oxide. Round or odd-shaped concretions are sometimes formed consisting of concentric layers of iron minerals such as limonite and goethite.

In this case the concretion consists of a hard limonite 'shell'. It appears that the iron compounds slowly migrated outwards from a central point, until crystallising as a limonite layer.