Environmental settings are the ones you've already been using...ie...according to altitude, slope, etc.  

Mix snow and rock, set the altitude and slope and then save it out with some name..(snock?:)

Then find your grassy material and mix it with the one you just saved.   A saved mat will load like a single one no matter how many mixes it has in it.  Then set the environment for that mix.

Then just adjust the mix slider left or right for the amount. 

Also, in the lower right is your mix blend....

And as rigul states above, you could just do it with layers and use those environmental settings....I'm just partial to mixes is all..