Holes in the Beach

Last day of our trip to the North Yorkshire coast and we went for a walk along the beach from Runswick Bay to Kettleness. About half way along there are loads of circular holes in a flat area of the exposed rock. They are all about 50cm in diameter and of varied depths.......I have no idea how these were formed...so any ideas from the good folks of Blipfoto would be gratefully received! Current theories include: Man made - holes are the result of the extraction of blanks for making millstones; Natural - action of the sea...but why so regular; Aliens - these are scars from the support feet on alien landing craft.

Also despit how I tried I couldn't get a proper set of 'Olympic' rings...best i could do is this upside down set.