Triassic To Permian

From the heights of the Cotswolds the great lowland of central England spreads out northwestwards - known as the Midlands for many centuries. The wide vales of the rivers Avon and Severn, which lie on either side of the great industrial conurbation of Birmingham, converge at Tewkesbury before flowing into the Bristol Channel. Stratford-upon-Avon is perhaps the most internationally famous of the River Avon's historic communities because of its association with Shakespeare, but from the point of view of industrial history there are also Rugby and nearby Coventry.

Geologically, the Avon flows over both Lias strata of Jurassic age and older rocks, which Smith knew as Red Marl and Sandstone but the strata soon became known as the New Red Sandstone. These rather unimaginative names did at least attempt to differentiate it from the more ancient Red Rhab and Dunstone or Old Red Sandstone, as it is more commonly known. However, as we shall see, there was at times a lot of confusion over their separate identities. The overall outcrop of the New Red Sandstone parallels that of the Jurassic strata from the south coast northeast to Durham and the North Sea. But from the Midlands northwards there is also a considerable northwestern extension to Lancashire and the Irish Sea known as the Cheshire Basin.

The uppermost and relatively younger strata of the New Red Sandstone are mostly mudstones, which can be difficult to differentiate from the Lias. The older and lower New Red Sandstone strata are predominantly sandstones, as their name suggests. In places the dark red-brown and occasionally almost blood-red sandstones outcrop at the surface to form low hills. Historically, the sandstones were widely used for building and many of the low hills are surmounted by ancient prestigious buildings such as churches, castles and the grand houses of wealthy landowners.

However, in Cheshire the deposits are interbedded with layers of rock salt and gypsum, which have been extensively exploited commercially. As a mineral formed by the evaporation of seawater, rocksalt is equally easily dissolved by rainwater, so that in fairly wet climates like those of Britain today, layers of rocksalt do not outcrop at the surface but may persist below ground. To begin with the rocksalt was mined by the ancient method of pillar and stall, whereby caverns were excavated deep below ground with the roof supported by pillars of rock left in place. Britain's last surviving rocksalt mine was still producing nearly 2 million tonnes of rocksalt a year by this method in the late 1980s. But nowadays most salt is extracted from the Cheshire deposits as brine. Hot water, pumped into the salt-bearing layers, dissolves the salt and the resulting brine recirculates to the surface where it is evaporated to recover the salt. The downside of the extraction process is widespread subsidence as the land sinks and closes the underground spaces. Large holes can suddenly appear that are big enough for houses to collapse into.

The occurrence of evaporite minerals such as salt and gypsum in the New Red Sandstone tells us that the deposits bearing them must have been laid down in tropical climates. The climate needs to have been hot and dry enough to repeatedly evaporate large, shallow bodies of seawater to form salt pans over a significant period of time so that substantial thicknesses developed (one of the Cheshire salt horizons is over 290 m thick). The relative age of the deposits in England proved very difficult to determine because of the lack of marine fossils. The resolution was first found on the continent and especially in Germany, where there are extensive strata of New Red Sandstone and associated strata.

Over the last 200 years a significant variety of fossils have at last been recovered from the Triassic deposits of Britain and record the kinds of life inhabiting the tropical landscapes of the time. Fossil bones are very rare, but many footprints and trackways have been found showing that vertebrate animals were much more common than their skeletal remains suggest. The plant record is particularly poor apart from some pollen and spores but there was a typical Mesozoic vegetation of clubmosses (lycopsids). seedferns (pteropsids), primitive conifers and cycadopsids and it was substantial enough in places to feed plant-eating reptiles such as the rhynchosaurs (1-2 m long) and occasional large, primitive sauropod dinosaurs such as Thecodontosaurus (up to 7 m long). There were even some of the earliest reptiles (Kuehneosaurus) capable of gliding 'flight', which presumably lived in the tree-sized plants from where they could launch themselves. Numerous rivers and lakes were important habitats for aquatic life ranging from clams to arthropods, fish and a variety of predatory amphibians that were somewhat crocodile like in body form. Haramiya, a tiny, shrew-sized and primitive mammal, lived a precarious life among the vegetation and may even have been nocturnal to avoid the numerous predatory reptiles and amphibians.

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