Many cockles found on the beach are fossils. They date back to the Eemian interglacial period. In those days, a warmer climate dominated the North Sea region. There were many species no longer found nowadays: poorly-ribbed cockles, spiny cockles, rough cockles and little cockles. Most of them closely resemble the present (common) cockles, but as a rule are much thicker. Due to the age, the shells are opaque and dark brown to black in color, and often worn.