How to get rid of white textures between layers on the terrain?

So I have a terrain material set up and it work s great apart from the occasional bleached textures between layers.

I have 3 hand made textures with height blending yet for some reason I get these weird textures showing up when they meet.

What is causing this?

How do I stop this? Is it something to do with each layers height and each layer in the material is using a height map to blend between them but for some reason the layers seem to blend when they shouldnt?