Yanoplathizo , don't know if you have gotten ananswer you like yet. Sometimes there is no complete shortcut in Photoshop. If I were doing this, I would use the quick select tool (if you don't have it, use the wand though it isn't as good IMHO for the task). Zoom in and clean up the selection a lot but don't spend tons of time on it. Create your selection and go into refine edges; alternately create a mask then go into refine mask. In either case you want to move the edge down into the mountains. One thing about organic images like this is that they are forgiving about changing the edges some. I made a selection of the sky, did selection>refine edge, added 14 pixels, no feather, no smoothing, 8? for contrast. This brought the selection into the ground, but made a fairly clean mask. Even then it needed a hard brush (because it is rock) on the mask to remove the slight blue halo. You might get cleaner results with a higher number of pixels but I didn't want to blunt the edges too much without seeing them.