Here's the problem for you ~ in the North Country the Romans didn't differentiate much between the Celts and the Germans in their reports, and ended up using the name of a Celtic tribe to categorize all of the Germans. Still, that was several hundred years before the beginning of the Dark AGes in 538 AD.
The area was depopulated ~ and not because they moved away. The Plague of Justinian did it's work. Northern and Western Europe took hundreds of years to recover.