The map above shows the four main earthworks within Cambridgeshire, which lie along a conspicuous line of chalk uplands which corresponds to the Icknield Way.
At the time they were built, the chalk downs were the only clear track between Hertfordshire and Suffolk, passing through Cambridgeshire. On the north side, the low-lying river courses were impassable in many places, and the lack of adequate bridges made long-distance journeys by this route troublesome, if not impossible. To the south, the clay soil was heavily forested and overgrown, and so it was only along the crest of the ridge that travel was possible for merchants, general travellers, and armies.
Of the four major earthworks, the Devil's Dyke is the most notable, at 7.5 miles (12km) long. The dyke is still up to 15ft (5m) high in places, while the ditch in front of it (to the south west) is 17ft (6m) deep and up to 65ft (22m) wide, giving any attacker a formidable obstacle, especially when it is realised that the ditch and dyke have probably been mellowed by time. In addition, the dyke is constructed from topsoil from the immediate area, and not from the soil excavated from the ditch, so it is thought that a secondary outer bank was constructed from this, which later partially collapsed into the ditch and/or was removed.
None of the ditches and dykes have been dated exactly, but excavations have shown that the underlying soil contains Roman pottery and some coins from the third century AD, conclusively disproving the earlier theory that the earthworks were from the Iron Age, and so contemporary with the few hill forts in Cambridgeshire. However, there is nothing to date the works themselves, and although an extensive Saxon cemetery was found beside the Bran Ditch which contained burials on an east-west alignment and which were devoid of grave goods (suggesting Christian burial, and a late date), this cannot be tied to the construction of the ditch, and is likely to be much later.
At the Fleam Dyke, excavations in 1921 suggested that the original bank was increased in size on two occasions, with large gaps between the improvements, but produced no dateable evidence from the bank itself apart from Roman building materials from a building known to be nearby.
The most likely explanation is that the ditches and dykes were built over an extensive period, perhaps starting with the immediate post-Roman period when the Saxons & Angles began their long fight for a toehold in England, and given the location and axis of the dykes, it is a reasonable assumption that they were a defence erected by the East Angles against the Britons. Indeed, an interesting comparison can be drawn betweeen these dykes and the major earthworks of the early period in Denmark to protect against the Germanic tribes around the same period - the very area the Angles migrated from.
Given the evidence from the Fleam Dyke, it can be assumed that the defences were kept in good order for at least three hundred years, and may even have been in regular use well into the Viking period, defending against the expansion of Mercia.
Whatever the reason for the building of the dykes, their scale and state of preservation make them worth a trip. Standing on the highest part of the Devil's Dyke and looking down into the deepest part of the ditch gives a vivid impression of the strength of the defences, and although the country around the Dyke is now rolling farmland to the south and drained fens to the north, it is still possible to see how well sited it was at a time when the country looked very different.