Mountain snow in Zbrush

Hi, I'm detailing some mountains in zbrush to use in a sky box and I want to create a layer of snow that is only on the horizontal surfaces. Is there any easy way of doing this, painting the snow by hand is pretty painful and looks rubbish. I've seen people do snow and sand as a shader in UDK, is there a way of doing this in zbrush.

I guess I'm trying to create a selection of only horizontal surfaces?

Thanks, Nick