Could be the base of a crane, a hand worked one at that. However I do have a slight issue with the fact the pole is tapered, there is a bearing area at the top but I'd need to see more before being certain. I doubt if it is the base for a waggon turn-table as they tend to have a narrow base (just a big peg really) and are not load bearing as the wheels around the circumference take the weight. There is a crane listed on the map, could be the base of that.
S4.