There was a "bug" in the PCM10 editor. After specific rechanging the lake waypoitns in the editor there were everywhere cliffs like this instead of a normal "beach". Never understood when exactly this happened. But obviously it`s still the same in PCM11 editor.
Theres is places in the world that looks like that. Here in Denmark, on the little island Fur there is a place like that. But it could be a big flooded quarry, like Lorkan says