The retreat left gravel, sand, clay and boulder deposits. Commonly extruded materials include metals, polymers, ceramics, concrete, modelling clay, and foodstuffs. Grapnels rarely have enough fluke area to develop much hold in sand, clay, or mud. Settlements made out of clay bricks were discovered in the plains. Marl is limestone with a high proportion of clay minerals. The clay, and bricks, were used to build walls. They lie under of waterlogged muskeg, of clay and barren sand. These may be made of glass, gemstones, metal, wood, shells, clay and polymer clay. Rocks that possess large amounts of both clay and silt are called mudstones. Mud or an adobe clay (daub) is covered outside. Tunnels are dug in types of materials varying from soft clay to hard rock. It represents a sandy lithofacies deposited in areas of stronger currents than the Gault Clay. Heavy clay and Ironstone deposits surround Banbury. Large deposits of a material commonly known as Leda clay also formed. There are more than twenty deposits of refractory clay and fireclay.