Saturday 18 November 2017

The geological history of the Longmynd

photo: Nigel Jones

The remarkable scenery of the Longmynd has a history going back over 560 million years ago to when the rocks, which form the range of hills, were first formed. Visitors looking at the crags on the hillsides will notice the obvious layered structure which is typical of the rocks which geologists call sedimentary. The layers are not horizontal but can be seen to be steeply inclined and this is evidence of violent earth movements which affected the area in the distant past and are an important part of the history of the formation of the Longmynd. This history is closely connected with the formation of the Stretton Hills to the east of the Longmynd on the other side of the Church Stretton Valley.

The modern concept of plate tectonics allows geologists to explain how continents move about the Earth’s surface as well as explaining the origin of volcanoes, earthquakes and mountain ranges associated with plate boundaries. The processes, which are happening today, have been continuous throughout the last 3,000 million years. Plate tectonics shows us that way back in the ancient Precambrian period around 570 to 565 million years ago the Shropshire area lay under a shallow sea, on the edge of a continent, close to the Antarctic circle at around 60 degrees south of the equator. The area lay on a plate boundary where one plate was overriding another causing crustal melting at depth and volcanic activity on the Earth’s surface. Around the edge of the continent, close to the plate boundary, were volcanic islands formed in what geologists call a marginal basin, similar to situations found in parts of the north-west Pacific Ocean today.

The volcanic islands, as part of what is called a volcanic arc, erupted great thicknesses of lava and ashes which geologists call the Uriconian Volcanics. These now form the well-known Stretton hills to the east of the Church Stretton Valley including, Caer Caradoc, Ragleth Hill and The Lawley, also The Wrekin further to the north-east. Although Caer Caradoc and The Wrekin look like extinct volcanoes, they are not! They are remnants of volcanic lavas and ashes formed by Precambrian volcanoes. We have never found the vents. Violent earthquakes affected the area at the same time and these produced great breaks in the Earth’s crust that we call faults. One of the most important faults in England produced at this time runs along the Church Stretton Valley between the Longmynd and the Stretton Hills and this is the famous Church Stretton Fault.

At the same time as the volcanoes were erupting, the weather was wearing away the islands and nearby land masses and rivers took the material down to the sea where it was laid down as layered sediments on the sea floor and nearby coastline. These sediments would harden to form the layered sedimentary rocks such as sandstones, shales, mudstones and conglomerates which now form the Longmynd. These layered sedimentary rocks can be seen in the sides of the main valleys of the Longmynd and amount to an enormous total thickness, estimated at between 6,500 and 8,000 m.

Within the great thickness of rocks which forms the Longmynd geologists can see that the earliest formed sediments were formed as muds, now hardened to shales, in a relatively deep sea which later became shallower. Volcanic ashes also occur in the early Longmyndian rocks and have been dated at between 570 and 555 million years old. Thick intertidal muds and silts accumulated (now shales and siltstones) and deltas grew out into the sea on which thick deltaic sandstones were formed. Towards the end of the period of time in which the sediments were formed, the sea became very shallow and the land even rose above sea level. At this time large rivers flowing off the land into a shallow sea laid down thick gravels which were hardened to form conglomerates which now form some of the highest areas of the Longmynd.

Some of the most remarkable structures found in the sedimentary rocks of the Longmynd are fossilised rain prints. These were formed when heavy showers fell on semi-hardened mud flats over 560 million years ago. The impressions left by the rain drops were then hardened by the sun and covered by later sediments to become permanently preserved.

The rocks of the Longmynd, formed as they were during the late Precambrian period, originated before life in abundance appeared on the Earth’s surface. Fossils are therefore almost absent from the rocks although some primitive jellyfish and algae have been recorded.

Around 550 million years ago the sedimentary and volcanic rocks were affected by great earth movements, which compressed and folded the rocks into a giant fold called a syncline. The steeply inclined layers of rock within this fold structure can be seen in the stream beds and hillsides of the Longmynd. The volcanic and sedimentary rocks were brought together by major lateral movements along the Church Stretton Fault from Late Precambrian times into the Devonian period around 400 million years ago. Important movements also occurred along the Pontesford-Linley Fault, a parallel fault 10km to the west.

During the vast interval of time between the formation of the rocks of the Longmynd and the present day, the area was alternately under the sea receiving further sediments, or an uplifted eroding land mass. During the last 560 million years the Longmynd area, as part of southern Britain, has moved slowly north from the Antarctic circle to our present northern latitudes, all due to plate tectonic forces and continental drift. There were times when the northward movement was erratic and we even moved back southward for short periods. The Shropshire area spent many millions of years close to the equator, as shown by the nearby coral reefs of Wenlock Edge, formed during the Silurian period around 430 million years ago.

Around 60 million years ago the Shropshire area, along with southern Britain, had moved northwards to be not far from its present latitude. The whole of Britain was uplifted at this time and erosion set in a subtropical climate. It was at this time that the South Shropshire Hills including the Longmynd and Stretton Hills began to appear with their characteristic modem shapes. The Church Stretton Valley was eroded at this time along the weakness of the Church Stretton Fault. An enormous amount of erosion has taken place during the last 60 million years to remove many of the sedimentary rocks of younger geological periods, which had covered the Longmynd area since Precambrian times.

About 2 million years ago Britain along with the rest of the northern hemisphere. was gripped by the Ice Age. During the Ice Age there were many times when great ice sheets and glaciers moved south over Britain and also covered Wales and the Midlands, but these glacial periods were also interspersed with quite warm interglacial periods. We know little about the early parts of the Ice Age over Shropshire, but there is a great deal of evidence in Shropshire and around the Longmynd of the last glacial period when ice sheets spread down from the Irish Sea into the Church Stretton Valley around 20,000 years ago. The level of the ice reached about 300 m above present sea level, and therefore the highest parts of the Longmynd stood above the ice sheets and were subjected to an intense tundra climate, and permanent snow fields almost certainly covered the highest parts of the hills.

When the ice started to melt, around 15,000 years ago, the large amounts of melt water produced by the melting snow and ice, as well as higher rainfall, eroded the eastern Longmynd and cut the very deep valleys which we see today such a Cardingmill Valley, The Batch and Ashes Hollow. The erosion of these valleys produced large amounts of sand and gravel which were washed down into the main Church Stretton Valley and mixed with the sediments left behind by the melting ice sheets.

The story of how the ancient rocks of the Longmynd were formed allows us to look back into the ancient geological past over 560 million years ago to when the world look very different to how it looks today. However, geological processes continue at their slow and steady pace as they have done throughout geological time, and in many millions of years from now the Longmynd may well have been worn away completely to form new sediments on a new sea floor, which maybe in future will form a new land mass as plate tectonic forces within the Earth continue to alter the positions of the continents and the oceans of the world.

Plate tectonic forces caused by the widening Atlantic ocean still affect Shropshire today as shown by the famous Bishop’s Castle earthquake of April 2nd 1990. This measured 5.2 on the Richter scale and occurred along the old Pontesford-Linley Fault line 10km west of Church Stretton, generating shock waves along the Church Stretton Fault.

Dr Peter Toghill, School of Education, University of Birmingham. November 2001.