(Image: Newsquest)
A man suffered a fractured eye socket and a bleed on the brain following a reported assault.
Detectives launched an appeal after the attack on Lower Clay Pits, Halifax at about 8pm on Friday.
West Yorkshire Police said the 34-year-old victim had been involved in a verbal altercation before allegedly being punched in the face by another man.
The force are keen to speak to anyone who witnessed the incident or has information about the suspect.
Police statement in full
A spokesperson for West Yorkshire Police said: "The suspect is described as a man in his 40s, between 5ft 7ins and 5ft 9ins tall, of large build, with wavy hair.
"He was wearing a white t-shirt at the time of the incident.
"Officers from Calderdale CID are keen to speak to anyone who saw or heard any part of this incident or anyone with information about the man involved.
"Anyone with information is asked to contact police via www.westyorkshire.police.uk/livechat or by calling 101, quoting crime reference 13260563372."