In 1838 construction workers building a new sewer under
Blomfield Street, on the course of the Walbrook, discovered very large numbers of human skulls, though very few other bones were present. Since that time, around 300 further heads have been found in the bed of the river, generally in the vicinity of the initial discovery. It is believed that the number of heads destroyed by development, or awaiting discovery, may be in the order of many thousands. Writing around 1136,
Geoffrey of Monmouth seemed to be aware of the presence of the remarkable number of skulls on the riverbed. His
History of the Kings of Britain explained their origin as resulting from a massacre occurring during a Roman civil war, the
Carausian Revolt of AD 286–296. In Geoffrey's account, a
Roman legion under
Livius Gallus, besieged in London, agrees to surrender to the forces of
Julius Asclepiodotus on condition they are given safe passage out of Britain. Asclepiodotus is happy to grant this, but his allies the
Venedoti moved on the captives, beheading them all in a single day. Geoffrey wrote that the river was afterwards named after
Livius Gallus, the leader of the beheaded Romans, as
Nantgallum in Welsh (the language of the native Britons), or in the Saxon English as
Galobroc, from which Walbrook was derived. More recently, the skulls have been dated mostly to the early 2nd century AD, and it has again been suggested that the skulls are the consequence of an anti-Roman rebellion in the 120s when London suffered a second major fire often called the Hadrianic fire. and Liverpool Street ) revealed a Roman burial ground which included graves scoured by the River Walbrook, and it was suggested that skulls might come from this. Other factors should also be considered here: the 'heads' or 'skulls' which have been recovered were actually crania (ie only the main/top part of the skulls - the accompanying jaw bones, which would have provided vital evidence for beheadings, execution or indeed massacre, were missing); human skulls are profoundly identifiable as being human and are more likely to have been spotted and collected during construction work - every single controlled archaeological excavation in this area has also uncovered other human bones, creating less of a bias towards 'skulls'; the crania recovered recently at Liverpool Street had a wide date range and therefore could not have amassed as the result of a single event (the 2nd-century bias noted above probably simply reflects the size of Roman London's population). The vast numbers of crania found across the Walbrook valley suggest that, in essence, there were probably several different factors at play. Some individuals may indeed have experienced a violent end. The majority of the crania, however, were likely to have been displaced from their nearby graves by a combination of gravel extraction (to enable a 2nd-century expansion to Londinium which required large amounts of gravel for new roads, yards and thresholds) and occasional flooding by the Walbrook itself. Roman gravel-extraction workers, in a similar way to 19th-century construction workers, may have created an unintentional bias in, respectively, reverential crania reburial and crania collection and it is that which has emerged to help to create such an intriguing phenomenon. ==Catchment and course==