I'm from The Netherlands. In geography class (some 25 years ago) I learned there is a geographical border going from approx south west Netherlands to north east

I'm from The Netherlands. In geography class (some 25 years ago) I learned there is a geographical border going from approx south west Netherlands to north east Netherlands. Imagine from Zeeland to Nijmegen to Groningen, approx. Above that border, everything gets lower. Under that border, everything goes higher. I don't remember the cause of it. I remember rivers bringing a lot of mud with them over time but not sure if related. Also Zuiderzee (after Afsluitdijk remade into IJsselmeer) had floods which is why Afsluitdijk as part of the Zuiderzeewerken was build in the first place. Also look up Mr. Cornelis Lely, the architect if interested. The other massive anti flood system is called Deltawerken, in Zeeland and Zuid-Holland due to the Watersnoodramp 1953.