High pore pressures occur in soils composed of fine silts or clays. Soils vary from coarse sands to tight clays or shet rock clays and shales. Iron, bauxite, manganese, clays, limestone, and silica are mined. The cements binding these grains together are typically calcite, clays, and silica. Soil, consisting of polyanionic clays and silicates, generally has a net anionic charge. The Southern Lowlands consist of Miocene and Pliocene sands, clays, and gravels. In the center, fine materials such as clays, plasters and limestones were deposited. Such use includes industrial construction with clays, silica sand, and stone. There are deposits of quartz sands, brick and expanded clays, sapropels and limestone. These include direct industrial building use of clays, silica sand and stone. Water is also bound in hydrous minerals, especially clays and opal. Experts debate a number of nomenclatorial problems with the classification of bentonite clays. These components are often generated in situ by heating various clays and limestone. Soil types vary from heavy clays to light sands. It later grew through acquisitions in refractories, clays, ceramic bodies and technical ceramics.