The Swiss region of Upper Valais is a German speaking region which is physically isolated from all other German speaking regions. The dialect spoken there is so different from other Swiss-German dialects, that they can only communicate in Hochdeutsch with other Swiss Germans (as a comparison, normally Swiss Germans from different regions speak different dialects but they are mutually intelligible for them). (I do not have any written proof, it's just what some Swiss-German friends told me)
The region of Upper Valais is only connected to the region of Berne through the Lötschberg pass and to the region of Uri through the Furka pass, both of those are very high and were difficult to come across before railways tunnels were pierced. On the other hand, the region is physically connected to the French speaking Lower Valais and the remaining of Romandie.
It makes sense that is would have been much easier for French speaking people to settle in that region than from Germanic tribes. Even after Germanic tribes would settle there, the intense contact with French speaking people and isolation from Germanic world should logically have taken effect.