The Upper Calcareous Grit, the last of the four upward shallowing cycles that comprise the Corallian Beds of southern England, is relatively enriched in iron minerals, having local developments of chamosite oolite mudstone and much more widespread deposits of sand and mud containing variable amounts of siderite and disseminated chamosite. The chamosite oolite mudstones have a restricted fauna dominated by oysters and probably accumulated in slightly hyposaline lagoons where the ooids formed from mixed iron-, alumina- and silica-bearing gels. Siderite was produced during diagenesis from iron carried on the surface of clay minerals. This intimate association with the terrigenous clay fraction means that siderite occurs in sediments deposited in a variety of environments ranging from offshore shelf to lagoonal.

The most important factor responsible for ironstone development was a very low rate of clastic supply throughout Upper Calcareous Grit times. The iron was probably derived by normal processes of weathering and erosion of sedimentary rocks exposed around the basin margin, but this cannot be conclusively proved and quite different iron sources may have been involved.