6.3 Reasons to leave or to stay
Depopulation started in the 3rd century in Friesland as well as in Groningen. Most modern authors consider coinciding socio-political factors such as tribal unrest related to the collapse of the Roman Empire as the cause of the large-scale emigration, rather than natural conditions (e.g. Dijkstra, Gerrets & Nicolay 2008, 309). However, if socio-political unrest were the prime mover of depopulation, it is hard to understand why only the terp region was abandoned and not the Pleistocene upland. Although the destabilizing socio-political events of this period may well have played a role, environmental causes should not be totally ignored.
If natural causes indeed played a role, a period of marine transgression at the end of the Roman Iron Age, which in the past has often been mentioned as a reason for the abandonment of the area (cf. Knol 1993, 19-23), can be excluded. People in the terp region had been accustomed to living in an environment that was regularly flooded by seawater for centuries. Their terp settlements were well protected against floods (Bazelmans et al. 2012). However, drainage of inland parts of the salt marsh area became increasingly problematic in the middle-Roman Iron Age, due to the high cap ridges that had formed along the northern coast. As long as the area had been drained well, floods did not pose a major problem. A permanently waterlogged landscape and prolonged periods of inundation, however, were much more difficult to cope with. This might well have been the incentive of the emigration that started in the 3rd century or even earlier. It probably was a combination of factors at which we can only guess, which subsequently made the inhabitants of terps in well-drained areas leave as well.
It may be asked why some settlements in Groningen, such as Ezinge, were not abandoned in this period. The answer to this question is partly related to the surrounding landscape of these settlements, which must have been relatively well-drained, but that cannot be the only reason. People from terps in well-drained areas in Friesland, for instance Wijnaldum, did leave during this period. A second important factor is the social environment. As from the first century AD, the population of the Groningen coastal area had been part of a socio-cultural network that also included northern Drenthe and that reached far into Niedersachsen. This network saved the remaining population of Groningen terps in the 4th century from social isolation. Friesland had only come under the influence of this network in the 3rd century, as is indicated by the new Driesum-style pottery. However, it was an already depleted population that adopted the new style; these people were not part of this network in the same way as their Groningen neighbours. Existing social networks in the large salt marsh area of Friesland must have gradually weakened because of the exodus that started in the 3rd century. The increasing influence from the east could not prevent their collapse or stop the emigration process.