Cormoran was the giant of St Michael's Mount, the tidal island that rises from Mount's Bay near Marazion. In the old Cornish tradition he and his wife Cormelian were said to have built the Mount themselves, hauling great blocks of white granite to heap up their island home — Cormelian forced to carry the stones in her apron, and (in one telling) dropping a green slab that still stands in the causeway when she tried to sneak an easier rock past her husband.
The story
From his stronghold Cormoran preyed on the mainland. When his hunger stirred he would wade ashore at low tide and seize the cattle and sheep of the local farmers, slinging whole oxen over his shoulder, until the whole district lived in dread of him. None dared face the giant until Jack, a farmer's son from Marazion, set out to be rid of him. By night Jack dug a deep pit on the side of the Mount and covered it over with sticks and straw, then at dawn blew a horn to wake the giant. Cormoran came charging down in a fury, blind in the early light, and tumbled headlong into the pit, where Jack despatched him with his pick — winning the treasure of the Mount and the title by which he is still remembered, Jack the Giant Killer. A heart-shaped stone set in the cobbled path up the Mount is pointed out to this day as the giant's own heart.