The marine inspector photographed the dock pilings each quarter. Barnacles colonized from the waterline down, the deepest row first where the current was calmest. By the third inspection the entire grid was crusted over. But the winter storm drove a log through the pilings, and the impact scraped the bottommost row — the oldest, thickest layer of growth — clean to bare wood. The rows above, flexible enough to absorb the shock, kept their barnacles. By spring the bottom row was encrusted again.