GIMP probably fits better for this problem.
I don't know exactly how he did the mountains, but it seems that he used a 25% hardness large brush, with opacity set to something like 60% to 80%, color black for the shadows and probably white for the light part. Paint, polish, erase, until he thinks that it's good, and then set the layer blend mode to Overlay.
Maybe adding some hsv noise and then gaussian blurring it a little.
Hope it can help you a bit (and hope I am right). I'll try something.