Thanks for all these kinds comments.
Actually, I tried to figure mountains and hills as they are created : a rolling of tectonic plate spreading on another one.
First, I draw the upper level, giving a high elevation (about 45-50)
After that, a layer of rolling hills is placed under the mountains and the brush spreads like rays from the center of a circle, overwhelming ...
Using the same parameters for the distortion relief (except elevation, reduced to 20-24).
Finally, the edges of mountains are filled with little patches of fuzzy circles.
Well, perhaps a little sketch would be more clear...
mouvements montagnes.JPG
Any comment ?