Creating Complex Natural Materials in Cycles - Blender 2.6 Cycles: Materials and Textures - page 177

• We then built the Separator , a node group outputting gray-scale values
which, connected to the Fac input of the Mix Shader node, work as a
"stencil map", separating the two different materials on the mesh surface
accordingly to the black and white values. The two gradient textures in the
SLOPE frame, mapped on the position and on the normals of the mesh
and then blended together by the Burn nodes, make the snow material
(the white color value of the "stencil map") appear more on the mesh's
faces that have a more horizontal trend than a vertical one. Thanks to the
Add nodes, driven by the exposed Snow_amount input and influencing
the gradient of the ColorRamp nodes, it's possible to also set the quantity
of snow (that is, of the white color in the stencil) on the whole object. The
mixed textures in the DENSITY frame make the separation line between
black and white more frayed and natural-looking.
In the preceding image, you can see the "mask" black and white values working as
a separator between the rock and the snow materials.