Comment by glennon

This is a water gap, a feature which is commonly formed from a river older than the mountain range. In combination with that older river, in this case, the specific location was caused by "subrosion (a general expression applied to karst processes influencing the course of a river) and..meltwater channels at low altitudes" (Rohde 1994, Martini et al 2002-page 285).

-- https://en.wikipedia.org/wiki/Water_gap

-- https://onlinelibrary.wiley.com/doi/book/10.1002/97814443042...

-- Rohde P, 1994. Weser und Leine am Berglandrand zur Ober- und Mittelterrassen-Zeit E&G Quaternary Science Journal 44(1): 106–133, DOI 10.3285/eg.44.1.10.