Shropshire under the sea

As a completely land-bound county, it’s strange to think that Shropshire spent much of its prehistory as part of the sea bed.

Shropshire has a fascinating and complicated geology, with evidence from eleven of the thirteen recognised geological periods. From around 600 million years ago until present time, fossils and rock formations show us that the ground we walk on was beneath the sea for more than 50% of the time of its existence!

The earliest geological period, the Pre-Cambrian, shows a violent beginning to our geology. A major fault in the Earth’s crust – the Church Stretton Fault – provoked a line of earthquakes and volcanic activity, forming the Wrekin (a thick pile of volcanic lava and ash).

Later in the period, Shropshire lay under shallow seas, depositing the sandstones and shales that form the Long Mynd plateau. No fossils mark this period (4,500 - 600 mya) – the only life on Earth was primitive bacteria. Shropshire was around 60^o south of the equator at this point.

These shallow seas were the nursery of the Cambrian Explosion – the sudden appearance of new life forms across the world.

On The Ercall you can clearly see the transition - pinkish rocks from when life was emerging as simple soft-bodied creatures which make poor fossils. These are overlain by grey rocks with the fossils of creatures with hard shells, including trilobites and the fossil ripples of an ancient beach. One of our reserves, Comley Quarry near Church Stretton, yielded Britain’s earliest trilobites from around 550 million years ago!

Shallow seas persisted over Shropshire through the following two geological periods, the Ordovician (495-443 million years ago) and the Silurian (443-417 million). The Ordovician saw the formation of limestones and sandstones, including The Stiperstones. Sea creatures such as trilobites and brachiopods abounded. During the Silurian, with Shropshire now around 25^o south of the equator, one of Shropshire’s best geological features developed – Wenlock Edge.

view from the Edge — View from Wenlock Edge

With shallow seas to one side of the Church Stretton Fault and much deeper seas the other, Wenlock Edge is the remains of an ancient reef stretching from Ironbridge to Craven Arms. Think South Sea Islands - the tropical seas were teeming with life, with the reef being a particularly rich environment. Brachiopods like chunky cockles, sea snails, sea lilies (Crinoids) and trilobites like big woodlice, some swimming and some crawling on the sea bed thrived in the shallow waters along with the reef-building corals and sponges, and their fossils are found in the Wenlock limestone, while graptolites are evident in the shales formed beneath the deeper waters to the west.

The Late Silurian and Devonian – 405-355 million years ago – saw the whole of Shropshire above sea level for the first time, though lakes and lagoons were still widespread. The Ludlow Bone Bed, from the latest division of the Silurian, produced fossils of the earliest British vertebrates, rather spiny, scaly, primitive fish – the appearance of these together with the disappearance of exclusively marine fossil graptolites and some brachiopods shows a change to freshwater conditions. The sea had retreated – but only for a while.

By the Carboniferous period, 355-290 million years ago, most of Shropshire was again under water. Shallow seas were fringed by equatorial forest and wide deltas.

Land plants which had first appeared in the preceding Devonian proliferated, and the decay of vegetation in the deltas eventually formed coal (later mined around Ironbridge and Telford). Meanwhile, at Llanymynech, limestone was still forming from the shells of marine creatures just below the tidal zone of a shallow, clear sea, which does sound like a nicer place to paddle than amongst the rotting vegetation at Coalbrookdale.

The period around 300-200 mya (Permian & Triassic) had Shropshire dominated by desert conditions. It lay about where the Sahara is now, and conditions were similar. But overlying the sandstones formed by the fossilised sand dunes we can see the last encroachment of the sea over Shropshire.

During the Jurassic, around 200 mya, shallow seas once again flooded parts of the county, and small outcrops of early Jurassic sandstones and shales appear around Prees, offering the first ammonites and belemnites for fossil hunters. If we had been there, trying to catch these cephalopods (modern ones include squid and cuttlefish) in our shrimping nets, we would have been paddling in seas shared by ichthyosaurs and plesiosaurs – ferocious marine predators up to 14 meters long – and hoping that the water was too shallow for them!