During the time when giants where to be found all over Cornwall, there lived in the Penwith area a particularly troublesome giant called Bolster, who was so named because of his habit of smothering his old wives with a bolster once he had grown tired of them.
Although a troublesome giant much inclined to trambling the farmers' crops and stealing their livestock Bolster had a weakness for a pretty face and became infatuated with a local girl by the name of Agnes. But the young maiden did not relish the attentions of the giant
she resolved to put an end to the matter.
Drawing the giant's attention to a hole in the cliffs at Chapel Porth, Agnes calmly asked the giant to prove his love for her by filling it with his blood. Bolster readily agreed as it was such a small hole and he was such a big giant.
Unfortunately for the poor giant the crafty Agnes had neglected to point out to Bolster that the hole actually drained into the sea, and as fast as Bolster's blood flew into the hole it emptied out into the ocean below the cliffs. So the giant Bolster slowly bled to death leaving behind the red mark that you can see on the cliffs today.
As for Agnes they made her into a saint in recognition of her deed.