Travelling along the Rhine I saw many vineyards and was deeply impressed. It is an amazing view to see where the vine dressers have to work – I would not go up their vineyards above the Rhine, they are so steep!
That is the reason why they built walls, they simply have to protect the ground. Otherwise it would be washed down to the river with the next rain shower.
One of this walls I show here, it is made of shale or slate.