During the Cambrian, Shropshire was flooded by the sea, after the Global Ice Age ended 545 million years ago. Thick layers of beach pebbles and white sand were built up against the sea cliffs that were once molten lava. During this time, the shallow sea played host to the huge explosion of new life which occurred during the Cambrian. Shropshire has some of the most historically important evidence in the explosion of life and this is referred to in the naming and dividing of the Cambrian period. Trilobites that are found in the county are internationally important for deciding how the Cambrian is divided into smaller segments of time.