The Hopewell Rock are near a fresh water estuary, so when the fresh water mixes with the salt water of the ocean, it rises to the top and creates a layer of ice. Since the tide takes 12 hours to come and go, and it's been so cold here lately (as low as -38 with the wind chill), it very quickly frrezes, leaving it's mark behind in layers as the tide drops. Tides in the Bay of Fundy reach over 25 ft, so at high tide the Rocks are a series of small islands.