Permian
Earth movements during the preceding Carboniferous Period gradually raised the land’s surface across much of the UK in what is known as the Hercynian Orogeny. This event raised the Harz Mountains in Germany, and further north the Mercia Highlands that once extended from Devon to the Wash. Former areas of well-vegetated tropical delta-marsh, within which the commercially important Coal Measures developed, became buried as a hot arid desert advanced across the area. It was under these conditions that the oldest rocks to crop out within the Tees Valley were laid down between 290 and 245 million years ago during the Permian Period.
Lower Permian
The lowest, and hence oldest, beds in the succession locally are a mixture of dune-bedded sandstones and coarse breccias that accumulated upon the flat desert plain. Occasional wind-polished rocks, known as ventifacts, can be found amongst the deposits, which are comparable to those accumulating in the Sahara today. A lack of fossils perhaps highlights the harsh conditions that existed during the sediment’s emplacement.
Upper Permian
Further orogenic activity to the south caused the land surface to buckle and fold forming a broad inland basin. To the north and east, a communication developed with the Zechstein Sea, which rapidly transgressed across North East England to occupy the former desert plain. Further subsidence meant that this new arm of the sea reached depths approaching 200 metres further east, though locally the area was close to a shoreline. This marginal environment was colonised by coral reefs, stromatolites, and a rich fauna of other marine creatures. Their remains combined with a restricted input of fine sediment blown from the nearby desert to produce beds of limestone. This reef environment was to be short-lived however, as a new period of uplift caused the English Zechstein to become cut off from the main water body. Conditions rapidly deteriorated in the isolated sea as evaporation of its diminishing waters concentrated their mineral content. During the final phases a sabkha zone developed, made up of hypersaline lagoons, pools of hot mud, and glittering beds of evaporites stretching across the desert.
The Upper Permian is typified by five such incursions, followed by evaporation, of the English Zechstein (EZ1-5) with the later episodes never attaining great depth. The resulting strata comprise various limestones and mudstones with intervening beds of evaporites. The latter became important commercially during the late 1800s when salt extraction on Teesside constituted the beginnings of today’s modern chemical industry. Later, anhydrite was mined in great quantities around Billingham, and potash along with rock salt is still extracted from deep mines, over a kilometre below the surface, at Boulby, near Staithes.
Fossil content within the Permian succession diminishes the higher up one looks, and this is not simply an effect of the harsh conditions locally, but is reflected within the fossil record worldwide. During what has become known as the Permian Mass Extinction, some 96% of all marine species died out never to return, with a lesser, though not insubstantial, number of terrestrial creatures joining them. The event is billed by geologists as the greatest extinction so far suffered by life on Earth. Life’s tenacity, however, never fails to amaze, and the survivors of this catastrophe would, over the next 40 million years or so, adapt and radiate into niches vacated by many of their predecessors to produce a whole new era of life on Earth.