Anglo-Saxon word for cross is "rod" (modern "rood"). Cameron's English Place Names gives Rudston as meaning "rood-stone" (also Radstone in Northamptonshire). What lies beneath the metal cap and why is it there - damaged by cross removal and cap set to prevent frost penetration? Some Christian sects liked crosses and others didn't. Perhaps Cromwell's boyos knocked it off - they did elsewhere. Is it not likely that any stone or cist remaining in a churchyard would have been blessed at some point in order for it to be allowed to remain on holy ground? Lot easier to sprinkle holy water on the monolith than smash it up and cart it away.