If its the Hartley just south of Seaton Sluice I can't see anything on Multimap or The 1850 OS at old-maps.co.uk so I don't think its old (could be wrong though!) I've passed there myself a few times and don't remember anything.

There were two chapels their in the 1850's so it might be something from the groups - re-erected.

Than again it might be a modern megalith who knows, I'm not likely to be going that way for a while.