 Carboniferous Period ( 359 – 299 million years ago ) rocks are represented by the Carboniferous Limestone that forms the Mendip Hills , rising abruptly out of the flat landscape of the Somerset Levels and Moors . The limestones are very fossiliferous , and contain evidence of the abundant marine life that existed at the time of their creation , including fossil crinoids ( sea @-@ lilies ) , corals and brachiopods . 
