Thanks guys. The snow needed to be flatter than this, so it's actually not my deliberate doing. It's displacement intersection-based, but the terrain has been eroded using Daniil's plugin, and thus (?) some areas are apparently not flattened enough. So, not a differential displacement mix.
You have to take into account that it's a huge landscape (see the three humpback whales) and the coastal foam extends 10m or so. With a calmish sea like this I didn't want it much broader.