THE SALT MARSHES of the Wadden Sea, silted up to well above sea level, are very fertile and rich in protein-rich grasses. There was ample scope for the inhabitants to provide themselves with food. It was a good place to live, on artificially raised mounds known as terpen or wierden. In the Early Middle Ages, this resulted in a high population density. By the eighth and ninth centuries, the salt marshes, covering an area of about 2,000 square kilometres in the provinces of Groningen and Friesland, supported a population of at least 40,000 inhabitants, and probably a great deal more (Knol 2019a). It was the good farmland that allowed this high density. The settlements – in Carolingian times at least a thousand of them – were continuously occupied or frequently re-occupied, which meant that the sites became accumulations of settlement debris and artefacts. The lime-rich clay, and deposits of farm manure, provided good conditions for the preservation of organic remains. In short, the coastal zone is a rich source for archaeology. Unfortunately, the circumstances in which many finds were recovered – during commercial quarrying of the fertile soil – mean that contextual evidence tends to be limited.

Lex Frisionum, written at the end of the eighth century at the order of Charlemagne, governing the territory of the Frisians, indicates that this territory extended well beyond the current provinces of Groningen and Friesland. It ran from the River Zwin (formerly the Sincfal – on the Belgian border) up to the Weser estuary in northern Germany. In the western Netherlands, the Frisians mostly occupied the coastal dunes; the marshy interior long remained of little significance. The dune zone was cut through by the great rivers Scheldt, Meuse and Rhine. Trading settlements arose around their estuaries (Dijkstra 2011). In these parts there were no artificially raised settlement mounds. Here the archaeological record is known mostly from archaeological excavations. The German coast between the rivers Ems and Weser does have some salt marsh between the Wadden Sea and the higher Pleistocene interior, but there is less of it, as in many parts the Pleistocene geology comes quite close to the sea.

There are barely any native written sources from the Frisians. Indeed they are limited to some sixteen runic inscriptions from Frisia itself, and a few more found elsewhere (Looijenga 2003; this vol.).