Grass frows over Dirt, Clay, Peat and will thus hide these
Grass also grows on sides of blocks, if the blocks are arranged in a stair-like pattern with other grass-growing blocks
Cliffs of grass-growing material only have grass on top, not on the side (but they will also break down on update, so should probably not be generated in the first place?)
https://plus.terrafirmacraft.com/Land#Gravity
Depends on: #7 , #8 , #11