Add a new layer, change the mode to Overlay 100%, and fill with 50% gray. Use the Burn Tool to darken the rocks more.
4. How to Add the Bird
Isolate the bird from the background and add it to the upper middle section. Use the Free Transform Tool (Control-T) to make it much smaller to create an impression of distance.
Create a Hue/Saturation adjustment layer to desaturate the bird.
Use a Curves adjustment layer to reduce the bird's visibility, making it fade out into the foggy background.
5. How to Retouch the Arch
Cut out the arch from the original image and place it above the stairs. Use a layer mask to erase the bottom and blend it with the existing ground.