The Lower Ordovician in the Caledonian Stavelot Massif (Belgium) contains a coticule member, probably Arenig in age, composed of red hematite-rich shales alternating with thin yellowish coticule beds. These coticules are fine-grained highly manganiferous metamorphic rocks mainly composed of spessartine garnet, mica and quartz. Similar rocks of about the same age occur in several localities along the Appalachian and Caledonian fold belts. The Belgian coticules are thought to have originated from marly sediments which had been deposited rapidly, probably by density currents, and in which CaCO3 had been diagenetically replaced by MnCO3. During metamorphism, spessartine garnets were formed from this MnCO3 and mica. Manganese and iron are thought to have originated from volcanic-exhalative processes.