Grimm north and south

Schmid 'Alteuropa und das Germanische' lists the following
correspondences in river names

Norw. Fala ~ Lith. Pala
Norw. Neta ~ East Pr. Nede
Norw. Vaka ~ Lith. Vaga

At the same time many see a correspondence in (Celtic(?)) Hercynia(n
Forest) ~ Germanic Fergun- (< *perk-un-).
It is unlikely that the Grimm shift has taken place both in the north
and the south of such a large area. One of the areas must have been
conquered by a Germanic-speaking group at a time when the Grimm shift
was still taking place, that is when the miltary aristocracy (found
in archaeology) had adopted that shift, but the 'natives' hadn't yet.
Which matches what Kuhn found for the Nordwestblock area.

Torsten