There is no masking in this photo. Looks compressed because it was shot with a 125mm lens on an aps-c sensor.
Yes I agree, obviously a single shot.
I think the reason the image (which has excellent composition BTW) looks like it is in two parts even though it isn't, is because you have quite a bit of midday haze visible on the distant mountains and not on the rocks closer to you. This can be a problem when stacking or compressing near/far elements within a single shot, especially in the middle of the day when the sun it at its zenith. The blue haze is made up of very small droplets of water surrounding even smaller particles of dust, because as the heat rises, the dust rises and attracts water out of the atmosphere etc. This atmospheric phenomenon becomes more apparent the further away you are shooting from the subject, as the depth of the air/haze you are shooting through increases and less so the nearer to the subject, where the air/haze is decreased relative to you.
You can sometimes eliminate some of the dreaded blue haze with a polariser, or try shooting early/late in the day when the sun is not so intense and the dust has settled, or another good time is after a good down pour of rain when the air has been washed and the dust is stuck on the ground and everything looks crisp and you can see for miles, or when it is very cold which will also work.