Create a landscape mesh with gaea or something.
Add a new blank material and go into the shader editor. Change the color to dark green or something so you don’t go blind.
Slope mask
Geometry node
Seperate XYZ (vector)
color ramp
Make this setup to make the slope mask. You will probably have to move the black slider to the right to see the effect start to work.
To see the mask preview, hook it into the Base Color input on the Principle BSDF node.
Using the mask
We can use a “mix” node with this mask to lerp two textures.
To add a new texture to combine, copy the bsdf node, select it, then press ctrl + shift + T (enable node wrangler add on for this hotkey) and select a texture set to make a texture. 
This will create a new shader inside our material. You can mix different shaders in one material if you have a mask like this.
Do this again for a stone shader.
Then add a Mix Shader node.
Connect your bsdf nodes into the inputs and your slope mask into the factor, and your materials should now “lerp” according to the slope mask.
Adjust gradient value in the slope mask for more or less falloff.
If you connect the “generated”
slot instead of the “UV” slot from the texture coordinate node on the rock texture you can get a more “world based” / triplanar tiling. This is useful if your rocks are mostly “protruding” / vertical. You will probably have to mess with the scaling but if done right you can get something like this.
That’s about it. great job