This is a cairn we found on the beach recently. We have been reading the wonderful D'Aulaire's Book of Greek Myths recently and discovered the story behind the first cairn.
Zeus' wife Hera was furious at Hermes for killing her servant Argus. She insisted that Hermes be tried for the crime by the minor and great gods on Mount Olympus. Each god who thought Hermes should be punished had to throw a pebble at Hera's feet. Everyone who thought Hermes should go free had to throw a pebble at Hermes' feet. Hermes defended himself well. Should he be punished for boring Argus to death? The pile at Hermes' feet was much larger than Hera's, so large that it completely buried him. It was the first cairn, and Hermes went free to do more mischief.
Read this story and others about Hermes. He is the funniest Greek god. He played a trick on his brother Apollo just a few hours after he was born!