Officers said the man had last been seen alive on the afternoon of Friday December 11.
Det Insp Harratt said: "The area where the man was found is extremely remote and is used by walkers, engineers and rangers for the park.
"What we know is he was last seen at around 4.30pm on Friday December 11 on the track near to where he was found and would appeal to anyone with information to contact us."
Police said the mystery man was a white, slim and had a receding hairline with grey hair to the side and back.
He also had a large nose that may have been previously broken and was clean shaven.
On the day he disappeared he was wearing a brown heavy jacket, blue jumper, and a white long sleeve shirt, with blue corduroy trousers and black slip on shoes.
Anyone with information is asked to call police on 0161 856 8972 or phone Crimestoppers anonymously on 0800 555 111.