Plus the Anthropocene within the Holocene, a proposed ‘new’ epoch that begins when human activities started to have a significant global impact on Earth’s geology and ecosystems)
During the Carboniferous epoch, 340 million years ago, the Underbath – part of the southern Peak District – was covered by a warm, clear, shallow sea full of microscopic shell-creatures.
On the sea floor were several coral reefs and beds of shellfish perhaps similar to modern-day cockles. There were also crinoids, animals related to starfish, that had stalks built of rings of calcium carbonate.
Over millions of years these creatures lived and died, gradually laying down a thick bed of calcium deposits from their shells up to a depth of 600m in places.
This deposit did not fill the sea because the sea bed gradually sank under the weight of the accumulations. It became the rock known as Carboniferous Limestone and it underlies not only the Underbath but the whole of the Peak District.
Throughout the period of formation of the limestone there was occasional volcanic activity and, at intervals, undersea volcanoes poured lava across the sea floor or injected it into the gaps in the beds of the compressed shell-creatures. This lava cooled to form layers of basalt, known locally as toadstone or t’owd stone ie. ‘the old’ or ‘difficult’ hard stone. (Not to be confused with toadstone, also known as Bufonite, a mythical gem thought to be found in the heads of toads, which was supposed to be an antidote to poison. These toadstones were not basaltic at all but the button-like fossilized teeth of Lepidotes, an extinct genus of ray-finned fish from the Jurassic and Cretaceous periods.)
Over millennia the greater landmasses moved causing significant changes in climatic conditions. The tropical sea covering the Underbath became more shallow. From about 300 million years ago, large rivers from the adjacent continents to the North began to drain into the sea and deposit silt, which gradually compressed to form rocks. The first layers of silt were fine and formed shale beds whilst later deposits were very coarse with large lumps of gravel in them and they laid down thick layers of sediment which hardened to form Millstone Grit. These layers eventually covered all the limestone.
As the sea filled completely, islands/atols of land formed and became richly vegetated, with trees and huge ferns. This was a fluid period, with the land sinking/rising/sinking and the huge delta rivers changing their course.
This movement caused the laying down a series of alternating shale and vegetable deposits which are now known as the Coal Measures. The whole era of rock formation, known as the Carboniferous period, lasted for 65 million years.
It was followed by an era when the Peak region was subjected to massive earth movements which raised and folded the rocks. This folding was not even eg. the rocks to the West were folded more than those to the East. The region was raised in a North-South line and resulted in the dome-like shape of the modern Peak District with the overlying rocks worn away until the limestone beneath was exposed.
At the end of this period the Earth’s crust sank and the Underbath was again inundated by the sea which deposited a whole range of new sedimentary deposits over it. Limestone often cracked under the combination of uplift and the weight of the overlying deposits, and molten rock (magma) was forced into the fissures. This was often rich in minerals, such as galena (lead sulphide), fluorspar (calcium fluoride), barytes (barium sulphate), calcite (a form of crystalline calcium carbonate) and sometimes copper, all of which crystallised as the magma slowly cooled. The result was the many mineral veins or rakes of the Underbath which have been mined for lead and other minerals since Roman times.
About 63 million years ago, at the start of the Caenozoic era, the area was raised again by more massive earth movements and most of the newer rocks which then covered the Peak District were gradually removed by erosion. It was at this time that the basic river pattern was laid down, and the rivers have continued to flow – carving and altering the landscape – in the directions then set down.
The most recent events which affected the geology of the Peak District were the Ice Ages. In some of these icy intervals, particularly the Anglian glaciation of around 450,000 years ago, much of the Peak was covered by thick layers of ice which scooped out hollows in the underlying rock. The melt waters from these glaciers would also have helped to form many of the caves now found in the local limestone.
The Underbath was not covered by ice during the last glacial period, which peaked around 20,000–22,000 years ago.
What can be seen today, the outward expression of millions of years of evolution, is the result of erosion, climatic changes and man-made impact.