Police are investigating after graffiti was scrawled over stone walls at Whitby Abbey and parts of the grounds dug up by opportunist metal detectors.
Staff doing a daily patrol of the grounds found the damage.
The name Jacob was written with what appears to be a wax crayon on a wall while the opening times and other sections of stone work were scribbled on.
There are also five separate sites of earth which appear to have been disturbed with spade marks clearly visible in the ground.
Staff at the abbey said they had not heard of metal detecting taking place in the grounds before and while signs could be replaced, the damage within the grounds was far more serious.
Delphine Jasmin-Belisle said: “Little things like that are frustrating. We can replace signage but for me as the supervisor of the site and having a background in archaeology, the metal detecting is far worse.
“There was some excavation in the 1920s and 90s but there are large parts that have not been explored and they are particularly dangerous. If someone was to go with a metal detector and have a look around with a shovel we don’t know what we could lose.
“They could have found a coin a visitor dropped last year or a ring from the 12th century.”
Whitby police say the incidents took place between Saturday 2 and Sunday 3 March.
PC Melanie Smith added: “Whitby Abbey is a historic property and it is beyond comprehension what anyone has to gain by damaging our heritage. Nor is it acceptable for anyone to enter the property and dig up the grounds for whatever purpose.”
In its policy on metal detecting, English Heritage refers to the Code of Practice for responsible metal detecting which says “it is illegal to use a metal detector without the landowner’s permission, and metal detecting on a Scheduled Monument is illegal without a Section 42 licence.”