Geological History

The remarkable scenery of the Longmynd and Stretton Hills 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 of the Longmynd will notice the obvious layered structure which is typical of the rocks which geologists call sedimentary. They are in fact hardened layers of ancient sands and muds laid down under water. 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 where the rocks are very different. The obvious crags of Caer Caradoc usually lack the layered structure of sedimentary rocks and are in fact volcanic lavas and ashes.

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 3000 million years. Plate tectonics shows us that way back in the ancient Precambrian period around 570 to 560 million years ago the Shropshire area lay 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 faulting ,and crustal melting at depth, and volcanic activity on the Earth' s surface. Around the edge of the continent, close to the plate boundary, volcanoes formed in what geologists call a marginal extensional basin, similar to situations found in parts of Indonesia today.

The volcanoes, as part of what is called a volcanic arc, erupted great thicknesses of lavas 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 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. The start of the volcanic activity was heralded by violent earthquakes which split the earth' s crust and formed longs lines of dislocation which 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 volcanic arc and nearby land masses ,and rivers took the material down to the sea which now covered parts of the marginal basin. The eroded material was laid down as layered sediments on the sea floor and nearby coastline. These sediments formed between 567 and 552 million years ago would harden to form the layered sedimentary rocks such as sandstones, shales, mud stones 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 7000 and 8000 metres.

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 567 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 of many kilometres 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 their 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 modern 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, further eroded the eastern Longmynd and cut the very deep valleys which we see today such as Cardingmill Valley, The Batch and Ashes Hollow. The initial formation of these steep valleys may have occurred during melt phases of earlier glacial episodes of the Ice Age in the last 300,000 years. 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 sands and clays left behind by the melting ice sheets.

The story of how the ancient rocks of the Longmynd and Stretton Hills 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 at depth along the old Pontesford - Linley Fault line 10km west of Church Stretton, generating shock waves along the Church Stretton Fault. The Dudley earthquake of September 23rd 2002, 4.8 on the Richter scale, probably was initiated along the old Malvern fault line.

These minor earthquakes which affect Shropshire and other parts of Britain from time to time do not cause any surface displacements either side of the faults. We shake, but we don't move, unlike the famous San Andreas Fault of California where movements of several metres after earthquakes can be measured. So we consider our famous Shropshire faults to be inactive nowadays, unlike the situation 560 million years ago! But, who knows? In 50 million years time we may be on a plate boundary again with volcanoes and California style earthquakes.

References:
Toghill, P. Geology in Shropshire, Airlife Publishing, Shrewsbury, 1990. 9.95
Toghill, P. Geology of Britain, Airlife Publishing, Shrewsbury, 2000. 29.95.
Paperback edition, 2002. 16.99 .

Dr Peter Toghill, Church Stretton. November 2002

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