Humberside Police is appealing for witnesses following the assault of a man on Yapham Road in Pocklington shortly after midnight on Saturday, April 6.
The man sustained a serious head injury following an incident where he had challenged men that were on his land illegally metal detecting known as nighthawking.
The men left the scene in a silver vehicle believed to be an Audi.
Nighthawking is a term used to describe unlawful metal detecting on farmland, archaeological sites and other areas of archaeological interest heritage, usually in order to steal coins and other artefacts for their historical and financial value.
A police spokesman said: “Nighthawking refers to the fact that such illegal activity is often undertaken at night to avoid detection and arrest.
“Although this is deceiving as it also occurs during the day.
“It can occur anywhere in the country, but counties such as the East Riding of Yorkshire are particularly vulnerable due to the rich heritage and large amounts of arable land.
“The removal of any object from land without the landowner’s permission may amount to an offence of theft.
“It is also an offence to metal detect on a Scheduled Monument without a licence or to fail to report the discovery of potential items of treasure.
“If you witnessed this incident and have not already spoken to officers please call us on our non-emergency number 101 quoting reference 16/44596/19.”