Most grounds are under the "nature" folder, but they're mostly stored as what we call blend materials. These have two textures you're meant to be able to combine smoothly (ex. grass to dirt, dirt to rocks, Foundry's concrete to piles of coal) to give the ground more variety. You can see a rough preview of this in the thumbnail. The downside to this is that some of the types of terrain involved don't have a corresponding plain material (grass, for one) that you can just apply to a brush and be done with it. You'd have to make your ground a displacement (which will not seal the map) and then Invert Alpha from the displacement dialog to swap to the other texture.