Hertfordshire, located north of London, belongs to the great shallow syncline known as the London Basin. Cretaceous Chalk is exposed in the north and west of the county, forming the Chiltern Hills. The chalk is not karstified and has no caves, but the soft rock was used for the digging of artificial structures. All listed sites are of this kind. The rest of the county is lowland covered by glacial sediments and so there are no naturel caves either.