Fen Causeway or the Fen Road is the modern name for a Roman Road between Denver in the east and Peterborough in the west. It's path covers 24 miles, passing March and Eldernell (near Whittlesey) before joinging the major roman north-south route Ermine Street west of modern-day Peterborough. It provided a link from the north and west of England to East Anglia.
It is possible that the route continued east of Denver to meet Peddars Way at Castle Acre, but the evidence for this is less certain.
The road is thought to have been raised above the marshy fens using gravel, with a width of up to 60 feet (18 metres).
References
T. Codrington, Roman Roads in Britain (1903)
