100 SEDIMENTARY ROCKS/Ironstones with mudstone and seat earth deposits (Figure 3) Palaeontological and mineralogical evidence indicates that these ironstones were formed during freshwater inundation Unlike non-marine clayband ironstones, there is an absence of early diagenetic pyrite, and the occurrence of coal balls (calcite–pyrite concretions) is an indication of marine incursion The ironstones typically form thin (less than 10 cm) sheets of less than 10 km2 extent and often change laterally into limestones with similar textural characteristics Bog iron ores, which occur as lenses of ferruginous concretions within peat deposits, are thought to be the modern analogues of blackband ironstones Claystone Ironstones Claystone or clayband ironstones have been the basis of the steel industry in many industrialized countries, largely because of their association with coalfields Essentially, they are accumulations of iron carbonates (usually siderite) that have replaced the non-marine shales of coal-measure cyclothems (parasequences) and occur as either thin sheets or, more commonly, layers of concretions (Figure 4) Occasionally these sheets may extend over several hundred square kilometres Normally, each concretion is unlaminated and does not contain high amounts of organic material, and the siderite grains are usually microscopic or sub-microscopic in size (less than 10 mm) Marine claystone ironstones are predominately rich in ankerite with pyrite, and production of siderite is suppressed Irregularly shaped sphaerosiderites (ball ironstones), which usually occur at the base of palaeosols, are composed of siderite cement in the form of distributed spherulites (0.5–1 mm in diameter) Ooidal Ironstones Figure Idealized stratigraphical column for blackband iron stones showing relative water depths of sedimentation, not to scale Ooidal ironstones are characterized by the presence of ooids and/or pisoids and are very diverse, with a Figure Idealized stratigraphical column for claystone ironstones showing relative water depths of sedimentation, not to scale