Joe Davies encourages us to travel back to a time and place that we can only now just imagine. The time is somewhere between 290 and 250 million years ago, a period in Earth’s geological history known as the Permian period.
At this point no humans or dinosaurs inhabited the planet, there were no historians to record the events of the day. All that we know about that time period has been pieced together from the evidence collected by modern-day scientists.
Artist’s impression of what it may have looked like 250 million years ago.
The beginning
250 million years ago, Earth was immensely different; the seven continents spread across the Earth’s surface today were preceded by a single ‘super continent’ called Pangaea, where all landmass was grouped together. This super continent stretched from the northernmost point of the planet all the way to its southernmost limit. Back then, a shallow landlocked sea called the Zechstein Sea sat around the Earth’s equator and today we now stand where this ancient sea once was.
This shallow tropical sea, believed to have been fed primarily by rivers of fresh water and was surrounded by a much larger outer sea, only separated by a narrow strip(s) of land which acted as a barrier, it is only because of this natural boundary that we have a story to tell.
We generally describe the Zechstein Sea as a dying sea, to conjure up images of the present day Dead Sea. However, in the early stage of its formation it wasn’t dying. In fact the first incursions of sea water were full of life and gave rise to muddy, fish-rich layers, evidence of which can be seen in the”Marl Slate” layer within many of the quarries located around East Durham.
Over millions of years the Zechstein Sea went through a series of cycles of drying up and reforming as sea levels rose and fell. To put the Zechstein Sea into perspective, it covers roughly the same area as the present day North Sea, beginning in the east of the UK and stretching towards the western border of Poland.
Artist’s impression of fish
The creation of Magnesian Limestone
As sea levels changed, this land barrier would be breached and the salty waters from the much larger outer sea would spill over in to the Zechstein Sea. The hot, dry air that blew across the surface would continually evaporate the water, resulting in a concentration of huge salt deposits, creating what some refer to as a ‘hyper saline sea’. This infiltration and evaporation process is similar to what happens currently in the Dead Sea, only on a much larger scale.
As the sea water evaporated, minerals crystallised out in a definite order, depending on their solubility, normally carbonates first (dolomite), followed by sulphates, gypsum, anhydrite and finally chlorides, including halite.
The milky yellow rock we see today outcropping at Castle Eden Dene and along the Durham Heritage Coast is a dolomite and known as Magnesian Limestone; it varies from other types of limestones in that some of the calcium component is replaced by magnesium. This East Durham topographical feature is known today as the Durham Magnesian Limestone Plateau, rising from the central Durham coal measures to almost 200 metres, and forms the stunning cliffs of the Durham Heritage Coast. Further to the east, and out into the North Sea, deposits of sulphates and chlorides formed the cap rock to many of the North Sea‘s natural oil and gas reserves.
Fossilised fish in section of magnesium limestone
Extinction of Life
The Permian/Triassic boundary, c.250 million years ago, marked the end of the Permian period and the largest mass extinction event in Earth’s history; during which 90% of all life was wiped from the face of the Earth.
It is widely believed that continental shift led to the breakup of Pangea and ultimately the formation of the continents that we recognise today. This ‘shifting’ resulted in the failing of the world’s ocean currents which, coupled with airborne pollutants from giant salt lakes such as the Zechstein Sea, had a catastrophic effect on both aquatic and terrestrial life. It was this period in Earth’s history that eventually cleared the way for other life forms to dominate, a mass extinction event that ultimately led to the age of the dinosaurs.