I thought this was part of the old Roman wall, but it isn't. It's a huge chunk of Hertfordshire puddingstone, a conglomerate sedimentary rock composed of rounded flint pebbles cemented together by a younger matrix of silica quartz. Hertfordshire puddingstone was credited in local folklore with several supernatural powers, including being a protective charm against witchcraft. It's usually found near river beds -- this one is next to the River Ver, close to the Kingsbury Watermill.