Dynamic texturing?

multipass or multitexture with different textures with alpha. Different value of alpha defines visibility of texture.
For example: We need to draw a terrain with grass and mountain.
Steps (for example):
1. Draw terrain with grass texture (no alpha)
2. Draw the same with mountain texture (with alpha). Mountain texture in alpha channel contains information about visibility of texture (such 0% level is fully visible and 100% is fully unvisible). Where you need grass to be placed write 0% in mountain alpha and if you want mountain, so place 100%. 50% will give you half visibility of two textures.
And so on...