Glad it worked out 
You can also mask in addition by certain landscape layers if you want to. Here is a test I did for the GDC kite demo where a tiling rock pile layer was used with displacement and the layer was used to mask tessellation to only where the rocks were:
The one issue is you sometimes need to “boost” your layer alpha before using it to mask or it can create cracks. I multiplied the layer alpha by 5 or so then clamped to 1 and it worked. The wireframe screenshot kinda sucks (different angle) but you can tell how it sort of meaders to match the rocks. No rock meshes all displacement here.