 It consists of marshy grasslands, lakes, pools, fossiliferous coal and shales.Both marshes- Cors Malltraeth and Cors Ddyga geology is characterized by carboniferous limestone, millstone grit, soft coal-measure shales, sandstone, beds of coal, and Permian strata.The Afon Cefni valley is long and runs almost exactly parallel to the valley of the Menai Straits.