Age: 411– 398 million years (Devonian, Lower)
The period marks a time when the world’s continents were colliding, eventually to form the supercontinent Pangea. This joining of the continent 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 Senni Formation is a unit that was formerly positioned within the Breconian Group. It is one of the few lithologies of this age that has been quarried for aggregate. The unit crops out in southwest Herefordshire at Black Darren and the Cat’s Back.
The Senni Formation overlies the St Maughans Formation. The unit is characteristically green in colour and is dominated by channelised sandstones. The Ffynon Limestone is a calcrete bed that forms the boundary between the St Maughan’s Formation and the Senni Formation. The Fynon Limestone has also been quarried as aggregate and is similar in character to the Bishop’s Frome Limestone.
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