About 340 million years ago, the area which is now the Peak District was covered by a warm, clear, shallow sea. The sea was full of microscopic shell-creatures, and on the sea bed there were several coral reefs and beds of shellfish some of which were similar to modern cockles, as well as many other types. There were also beds of crinoids, animals related to starfishes, which had stalks made up of rings of calcium carbonate. Over a long period, millions of years, these creatures lived and died in this area, gradually laying down a thick bed of calcium deposits from their shells up to a depth of 600m in places – sometimes these have now been eroded in strange formations like the ancient coral reefs around Chrome Hill.
This did not fill the sea because the seabed gradually sank as the deposits were laid down. This is now the rock which is known as Carboniferous Limestone and this rock lies under the whole of the Peak District. The sea was also a centre for occasional volcanic activity and at intervals undersea volcanoes would pour out lava across the sea bed, or inject it into the gaps in the beds of compressed fossils. This lava cooled to form layers of basalt (known locally as ‘toadstone’).
As the sea became more shallow, conditions changed. From about 300 million years ago, large rivers from adjacent continents to the North began to drain into the sea and deposit silt, which was gradually compressed to form rocks. The first layers of silt were fine and formed rocks now known as shale as can be seen on Mam Tor, but later deposits were very coarse with large lumps of gravel in them and they laid down thick layers of sediment which was hardened to form Millstone Grit, as can be seen on the eastern edges at Stanage, or in the west at Windgather. These layers eventually covered all the Limestone.
As the sea filled completely, islands of land formed and became richly vegetated, with trees and huge ferns. However, in this period the land was constantly sinking and rivers were changing their course, so areas of land often would subside and become covered in water again. This laid down a series of layers of alternating shale and vegetable deposits which are now known as the Coal Measures.
This whole era of rock formation, which is known as the Carboniferous period, lasted for 65 million years. It was followed by an era when the whole region was subjected to massive earth movements which raised and folded the rocks of the area. This folding was not even, for the rocks to the West were folded more than those to the East, and even as the folding took place the rocks at the top of the arch were being eroded by the elements. The region was raised in a North-South line which resulted in the dome-like shape of the modern Peak District and the rocks were worn away until even the Limestone beneath was exposed. At the end of this period the Earth’s crust sank in this area and the whole region was covered by sea which deposited a whole range of new rocks over it.
During this period the rock (especially the limestone) often cracked under the pressures and molten rock (magma) was forced into the fissures. This was often rich in minerals, such as galena (lead sulphide – grey metallic coloured rectangular crystals), fluorspar (calcium fluoride – translucent white crystals, sometimes coloured blue or yellow), barytes (barium sulphate), calcite (a form of calcium carbonate – white crystals) and sometimes copper, all of which crystallised as the magma slowly cooled. The result was the many mineral veins or rakes which are to be found in the limestone areas and which have been mined for lead and other minerals since at least 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 covered the Peak District were gradually removed by erosion. During this time the basic river pattern was laid down in the area, and the rivers have continued to flow in the directions then set down despite subsequent changes to the landscape.
The most recent events which affected the geology of the Peak District were the Ice Ages, which occurred from 100,000 years ago to 10,000 years ago. In some of these icy intervals much of the Peak was covered by thick layers of ice which scooped out hollows in the underlying rock. The melt waters from the glaciers also helped to form many of the caves now found in the local Limestone. Herds of wild animals roamed the area and their remains have been found in many of the local caves – Buxton museum has a fine collection of these.
Much of the modern landscape depends upon the type of rock lying beneath. Limestone has fissures and is slightly soluble in water. This means that the rivers have been able to carve deep, narrow valleys and have then often found a route underground, creating caverns and leaving dry valleys behind. Sometimes, as at Cave Dale and Winnats Pass near Castleton, the caves have collapsed, leaving a deep, narrow gorge.
Millstone grit on the other hand is insoluble but porous, so it absorbs the water, which often seeps through the grits until it meets the less porous shales beneath, and then springs emerge. The shales are friable and easily attacked by frost, so they often form areas which are vulnerable to landslips, as on Mam Tor. Both the grit and the shales are less hard than Limestone, so the rivers have worn much wider valleys in these areas. At the top level there are still some Coal Measures in the Peak, and these were mined for (very poor quality) coal until the beginning of the 20th century. The remains of these can be seen on Axe Edge above Buxton.
The different chemistries of the various rocks encourages different plants to grow upon them. The Limestone regions were covered with trees until a few thousand years ago, but are now mainly grassland, supporting a wide range of abundant flowers and plants. The acid soil and wet climate of the upland gritstone areas has given it its own unique landscape.
Information taken from peakdistrictinformation.com