Sandhopper (Talorchestia megalopthalma) puzzle.

• This is the burrow of a sandhopper (beach flea), a crustacean that burrows along beaches. As a wave comes in, water would cover the burrow, and as the wave recedes, the water leaves behind erosion gullies as seen here. Question: which side (left or right) in this photo is the lower side (sea side) of the beach? Why?
  • Answer: left. After the wave receds, water still covers the burrow column, and the sandhopper below would expel the water out of the opening of the burrow, which is slightly below the surrounding sand. The expelled water forms a small crater.
  • On the left, lower side, the rim is intact after most of the water overflows the little "dam" and back down to the sea.
  • On the right, higher side, the expelled water flows back into the burrow with more force, creating the gullies.