Summary: Concretionary siderite horizons are quite common in massive clay sequences. One such horizon, from the Westphalian of Yorkshire, has been studied in detail. Two iron-rich carbonate minerals occur together although they cannot be distinguished in thin section on account of very fine grain size. One is much richer in magnesium (pistomesite) than the other (siderite). The latter is rela-tively rich in manganese and the heavier stable carbon isotope 13C whereas the former carbonate is richer in calcium and 12C. The most important iron source is thought to have been hydrated iron oxides originating in soils. Much of the carbonate carbon started as organic molecules. The siderite appears to have formed earlier than the pistomesite. The stratiform character of these deposits appears to reflect siltier horizons in the mudstones, which presumably channelled pore water migration during compaction. This is probably why such carbonate horizons were formerly believed to be of simple sedimentary rather than diagenetic origin.