The most simple way of masking is to use different Colour Adjusts and to subtract the results.
This way you have at least an overall, a bottom and a top-mask.
Using '-1' as input of an ArcCos gives you Pi. Multiplying this with any mask may let you give some Sin-controls over the masks.
Using the Sin-functions you can get Spires with a wall outside (like spire growing out of a crater). This is very difficult to mask, though.
Another thing that I pm-ed you is quite delicate and should be apllied before any other displacement ,-)
Volker