The stone which stands in the field to the left is described on the National Trust HBSMR (ref. 93535) as a 1.1 metre high undressed granite pillar which serves as a rubbing stone for cattle.
A logan rock or stone is one that, well, rocks. There is one here, it is clearly visible from the Porthcurno side of the bay, but in the short time we had to explore we could not find it close to hand.