Your best bet here is to concentrate seriously on the altitude-dependant materials. You can arrange them all and mix them any way you'd like and have darker materials at the bottom, in layers, slowly getting lighter as the material reaches the top of the terrain. It's really pretty complex at first, but if you look in the material editor after mixing two materials, there's a check box for altitude. From there on it's a matter of going through each material and determining how it interacts with the others, by adjusting the percentages slider, as well as the other controls. The good thing is, it's pretty easy to catcj on as to how this all works, so just experiment with it and you'll see. if you have any specific questions about it, please ask. Also, I made a material called "Real Shoreline", which is pretty much what it says... I could send it to you if you'd like to see it.