Raise your slope constraint up higher and lower it's fuzziness value, Also take the Alpha boost up to 100 i used the grass on top of the rock layer  so i use the slope constraint to help break up the two mats. you're not constrained to having the bottom layers as the bottom mats, you can set the position of where it actually lies with the Altitude Setting. As you can see with the grass settings.

Depending on the size of your terrain you might have to play with the scale, it depends on the material as to whether you increase or decrease, it's somethign you have to play with.

This is a quick render and some settigns (don't have enough time to play with it right now).