Hey, sorry, this got lost in the shuffle of my laptop being stolen. I took a look this morning, and it appears to be some interaction with the height map blending of the height maps producing the undesirable look. I notice that if I adjust the interpolation contrast, the shape of those edges changes considerably- I also noticed that if I manually paint on the layers instead of using the brush's auto mode, I tend to not have those issues either (though they can be created). My best guess is that the auto mode, which attempts to paint on the lowest weighted layer, is effectively causing it to flip back and forth between the textures rapidly, and because of the way the height maps are interaction you end up with these kind of "left over" bits of grass creating a channel between the areas. A bit hard to explain, but I hope it helps..