seems also that there is a way to give the surface a bump
Yes, that is what we're talking about when we say "normals". The "bumpiness" is stored in a texture called normal map. They're usually stored with the suffix _n, pairred to another texture- ie.
File.dds, File_n.dds - and ar eeasily recognisable as they appear a weird bluey-green-pink hologram like image.
The snow texture the pixelshader uses though is NOT normal mapped, that is defined by the normal map of the surface it is resting on, ie the boulder's. which happens to be the same normal map used in the snow distribution calaculation.