Concretions from Upper Cambrian sediments in South Wales can be divided into two groups of different ages. An early group (type) are characterised by the association of septarian structures and deformed laminae; whilst later (type Ha) concretions developed both cone-in-cone structures and parallel laminae.

Determinations of the cement content of individual concretions revealed centrifugal variations related to the porosity changes within the sediment during concretionary growth. Growth of type-I concretions began in sediment of approx. 70% porosity and continued until compaction had reduced the porosity to less than 40%. Later, type-IIa concretions began growth in sediment of approx. 30–40% porosity and experienced only slight compaction over their growth spans.

Liassic concretions from a prolific concretionary horizon in Dorset developed parallel laminae, but grew early in the absence of compaction (type IIIb) concretions at this horizon grew within the sediment during a pause in deposition.

Evidence is described for distinguishing between concretions formed in systems open or closed to seawater. It is concluded that Cambrian concretions grew in closed systems, but the early Liassic concretions may have developed in a partially open system.

A model is offered to explain the closed system growth of prolific concretionary horizons. The Cambrian concretions often proliferated along 2–3 cm thick siltstone units, which it is suggested were more permeable than the surrounding mudstones and therefore became pore-water migration paths. Along these migration paths concretionary growth may have relieved the supersaturation of mobile pore waters.