Chapel Porth on the North Cornwall coast lies at the bottom of a long, narrow valley and is reached down a narrow lane. Steep-sided rugged cliffs loom up on either side and Atlantic breakers crash against the rocky beach. At low tide the sea leaves behind a huge expanse of sand. Walking along the tide line it seemed to be lined by flat, black stones. On closer examination they turned out to be velella, also known as sea raft, by-the-wind sailor and purple sail.