Age: 416 – 397 million years (Lower Devonian)
The period marks a time when the world’s continents were colliding, eventually to form the supercontinent Pangea. This joining of the continents into one large landmass leads to the appearance of the first extensive terrestrial rocks in Herefordshire and Worcestershire. They are traditionally known as the Old Red Sandstone.
The St Maughans Formation is a unit within the Early Devonian of the Old Red Sandstone and one of the few lithologies of this age that has been quarried for aggregate. In Worcestershire the Formation outcrops on the high ground to the west and south of the Teme Valley, and also occurs in a small patch in the Trimpley inlier. In Herefordshire, intraformational conglomerates and calcretes are more resistant to erosion and form the higher land of the Bromyard Plateau and the “cornstone” hills of central Herefordshire. The unit was formerly known as the Ditton and Woodbank Groups. The Hackley Limestone crops out in the Bromyard area. It is a mature calcrete band with similar characteristics to the Bishop’s Frome Limestone of the Raglan Formation.
St Maughans Formation
The St Maughans Formation is made up of red/purple/grey mudstones, sandstones, intraformational conglomerates and calcretes, deposited from a braided stream system which ran over a vast, flat, arid landscape. The unit is typified by cyclic sequences moving from an erosive base with basal conglomerate, up into finer siltstones and eventually calcrete with carbonate nodules. Sandstone lenses infilling river channels and other fluvial features of seasonal streams crossing a semi-arid land surface can frequently be seen. These rocks have often been quarried, both for aggregates and for building stones. The harder bands, such as the intraformational and calcretes have been used for road stone.
The Hackley Limestone was formed by the precipitation of calcium carbonate in the St Maughans Formation and its overlying soils. This resulted from the movement of ground water in the unsaturated zone between the top of the water table and the land surface. Fluid movement brought soluble calcium carbonate from lower levels and deposited it through evaporation, close to the ground surface, forming a limestone deposit within the rock and soil called a calcrete. If the calcrete development is sufficiently prolonged, as in the Hackley Limestone, the calcrete forms a greater percentage of the rock and is said to be more mature.
Great Corras Quarry, Herefordshire (St Maughans Formation)
Hackley Farm 1, Herefordshire (Hackley Limestone)