Terrain Painter multiple textures

Hi, I'm trying to paint multiple terrain textures in Torque 3D. I have our base terrain built and have a base texture across the whole terrain but now want to built up the texture so that it has textures overlaid on top of the base layer. For example I want our base layer to be rock but on top of that I want to add a slight grass texture and then on top of that an other texture but when I paint over the rock base it is replaced by the new texture.

Is there a way to build up the torque textures on terrains that allow textures to be placed over each other rather than replacing the previous texture?