The climate in Denmark and southern Scandinavia becomes cooler and wetter during the Pre-Roman Iron Age (from the fourth to the first century BCE), limiting agriculture and setting the stage for local groups to migrate southward into Germania.

At around this time people begin to extract iron from the ore in peat bogs.

Evidence of strong Celtic cultural influence dates from this period in Denmark, and in much of northwest Europe, and survives in some of the older place names.