The next time you are visiting one the beaches of the Champlain Islands and you find a smooth, flat black rock with white crossed stripes, pick it up. You will be holding a piece of ancient geologic history.
The rock is Iberville Shale and it was created 485 to 445 million years ago during the Middle Ordovician Period when "Vermont" was located in the tropical latitudes. Silt that was layered in a tropical sea was compressed over time into the dark shale.
The white lines or veins came later when the compression forces that created the Green Mountains (450 to 350 million years ago) drove calcium calcium carbonate into the fissures of the shale forming calcite crystals.
Eventually through uplifting, erosion and wave action, the shale came to the surface, broke from the main rock and smoothed into the beautiful rocks that we hold in our hands and skip across the surface of the lake.
