Here is a screene what it could look like:
Here's the cap:
The problem is: There is no real Layer Mask function that works
I tried to work around the problem with additive Layer Effects, I'm looking for a simple white-to-black alpha mask changing the Opacity of the Layer below.
White - Grey - Black
100% - 50% - 0% Opacity. You get the idea ;)
So when someone tells me how this can be done or if someone writes a plugin I'm going to make some tutorials about landscape creation. Deal? :D