Hertfordshire puddingstone

Hertfordshire puddingstone is a conglomerate sedimentary rock composed of rounded flint pebbles cemented together by a younger matrix of silica quartz. The distinctive rock is largely confined to the English counties of Buckinghamshire^[1] and Hertfordshire but small amounts occur throughout the London Basin.^[2] It is quite commonly found in fields in and around Chesham, where pieces can be seen as boundary stones and in rockeries. Despite a superficial similarity to concrete, it is an entirely natural silcrete. A fracture runs across both the pebbles and the sandy matrix as both have equal strength unlike concrete where the pebbles remain whole and a fracture occurs only in the matrix. Like other puddingstones, it derives its name from the manner in which the embedded flints resemble the plums in a pudding.^[2][3] It forms the local base of the Upnor Formation of the Lambeth Group (lower Eocene, 56-55 million years ago).