The following text was inspired by a visit to the area led by Dr. N. Chidlaw of the University of Bristol, on the weekend of 22^nd/23^rd March 2003.

A look at the geological structures and chronology of the rocks around Church Stretton, The Wrekin and The Onny Trail, Shropshire.

The underlying rocks of the area are primarily late Pre-Cambrian in age; these are the eroded remnants of volcanoes and the magmatic bodies associated with them. The rocks cover the Sturtian and Vendian periods of geological time at 800 - 610 and 610 - 570 Ma respectively, by careful radiometric experiments on them, some are seen as early Cambrian also. These are the Uriconian Group and include basalt, andesite and rhyolite lavas and tuffs, with associated dolerite and granophyre intrusions in the form of dykes and sills. Other periods with rocks in evidence in the area are Cambrian, Ordovician, Silurian and Carboniferous. Other geological periods are not represented at the surface, though may well be at depth. The whole area is covered by a thin layer of Pleistocene material, which was left after the last Ice Age, and more localised, recent erosion.

When the Uriconian rocks of southern Britain were being formed, the area lay on the northern edge of a large continent, to the south, or a micro-plate at the edge of it, between 60 and 70^o south of the equator. To the NW a deep ocean received sediments from this continental block, which was buoyed up by rising magma from the ocean's subducting plate. The magma caused volcanoes to form on the continental block and the injection of various igneous bodies and forms. The subducting plate, plunging obliquely below the S plate, caused rifting and faulting, and major earthquakes within it, so that here also deep-rifted basins allowed much marine water in. Uplift and erosion around these basins then filled them with sediments and ashes from island arc type volcanoes. Due to the subduction of the oceanic plate, high pressures and temperatures were involved. This has given rise to the alteration of existing rocks to schist and gneiss. In the area they are known as the Primrose Hill Gneiss and Schist, which have not been dated. However, the Rushton Schist has been dated to late Sturtian at 667 Ma.

With the continued growth of the Uriconian Mountains and basins, their erosion added to the growth of the Longmyndian Supergroup rocks, which also formed in late Pre-Cambrian times. The sediments filled up basins along fault lines, running NE-SW, within the margins of the S plate, one of which was in the area of the present day Welsh Borderland Fault Zone (WBFZ). These huge deposits initially kept up with the subsiding basin. However, over time this also became filled so that changes in environment can be detected in the rocks, from deep-water turbidites, to near shore conglomerates and deltaic-estuarine conditions.

At some point towards the late Pre-Cambrian, the subducting plate squeezed and faulted these Longmyndian rocks into near vertical beds. The Longmyndian Supergroup is made up of the Wentnor Group, to the west, and the Stretton Group, to the east. The Stretton Group are faulted up against the Uriconian Volcanics to the east of the Church Stretton Valley, in a synclinal form. The Wentnor Group are faulted onto the Stretton Group, also in a syncline, but over the western limb of the Stretton Group. This brings the core of the Wentnor Group to the surface through the small village of Bridges, to the west of the Long Mynd. This means that the Longmyndian as a whole, lies between the faults of the Church Stretton Valley, in the east, and the Pontesford Lineament to the west. With all of this faulting and movement of vast blocks of continental crust, injections of magmatic fluids occurred in the form of basalt, dolerite, lamprophyre and granophyre.

As stated previously, there was a large ocean to the N and W. In this ocean the sub-continent of Baltica began to fault away from the northern continent of Laurentia. This N continent straddled the equator and included NW Newfoundland, N Ireland, Scotland and the north of England. Between these rifting blocks the Iapetus Ocean began to open. Baltica would move aside opening the Iapetus further and, much later, be once again involved with that ocean's closing.

However, our S micro-continent, which included SE Newfoundland, S Ireland, Wales and S England, was now part of the larger continent of Gondwana and we have arrived at the Cambrian period. This saw a huge expansion in forms of marine life with hard body parts, which also gave the ability to be preserved in the fossil record. Prior to this life forms had evolved but with only soft body parts. Even with hard parts fossilization is a precise operation, and the sequence of events, especially rapid burial, must occur on a creature's demise. The bulk of animals that are fossilised from these times are trilobites, brachiopods and graptolites, all being marine. There do not appear to be freshwater types as there are no land animals or plants either. They evolved much later.

The Cambrian period ran from 570 Ma to 510 Ma and after the Longmyndian rocks had been faulted, as they lay on an angular unconformity and a conglomerate. This conglomerate at the Ercall Quarries, SE of Shrewsbury, has clasts of rhyolite in it, Uriconian in age, and is some 7 metres thick. The bedding planes of the conglomerate and the Wrekin Quartzite above it dip to the SE about 40^o , so later movements also have affected these. Breakdown of the bedding planes within the conglomerate can be seen, and may be due to ash bands separating them.

The Wrekin Quartzite is formed of small rounded grains of quartz, is very clean, with no clay or detritus and would probably have formed in strong underwater currents. On some bedding planes there are fossilised ripples. The Cambrian through this part of Shropshire is only partly represented, as several unconformities have been mapped. This is due to the instability of the area through plate tectonics; certainly there would have been movement on the fault zone, both laterally and vertically. These rocks show a transgression of the sea towards the SE, with the conglomerate forming the shoreline initially, followed by the shelf sea deposits of the quartzite, with it's attendant ripples. As the sea deepened, the Lower Comley Sandstone and then Limestone were deposited. The limestone had some of the first Cambrian fossils in it, trilobites and brachiopods. Further tectonics, uplift and erosion saw another angular unconformity prior to the Upper Comley Sandstone deposits of the Middle Cambrian. Deepening of the basins towards the end of the Cambrian is reflected in the Grey and Black Shales of the Late Cambrian, where some calcareous shell bands are found.

The tectonic situation was much the same as before. However, the ocean to the N could now be called the Iapetus Ocean, as Baltica was now moving further away from Laurentia, opening the seaway between that continent, Laurentia, and the S continent of Gondwana. Rifting also began between Baltica and Gondwana so that a new sea Tournquist's Sea, opened also. With these movements occurring further adjustment to Gondwana was bound to happen as it did when our micro-continent of S England etc. faulted away from Gondwana forming the Rheic Ocean. This new block is known as Avalonia. The two parts of Great Britain, north and south, could have been anything up to 6000 Km apart at that time. This would have been the situation by the late Cambrian. By early Ordovician, subduction started on both sides of the Iapetus and thus began the closure of that ocean. As Iapetus closed so did Tournquist's Sea, so closing Avalonia to Baltica. It is believed that Laurentia stayed fairly stationary over the equator, whilst Avalonia and Baltica moved northwards towards it from the south. These two sutured about end Ordovician times, and carried on in the same direction to eventually close with Laurentia much later.

With the two halves of the UK closing on each other and igneous bodies adding ashes and sediments from island arc activity, nearly 4.5 Km of material was added to basins to the west of the Pontesford Lineament. Igneous intrusions were also injected. To the east barely a quarter of those sediments are seen, where uplift caused erosion in S Britain, which by now was just 40^o S of the equator.

By 510 Ma the Cambrian had been replaced by the Ordovician, which lasted to 439 Ma, in the area this is seen as a non-sequence. This is where a period of uplift leaves a break in deposition, so that although the Ordovician is deposited elsewhere, at an earlier time, here in Shropshire it is first seen as the Shineton Shales. These sediments must have kept pace with a subsiding basin, as they are nearly a kilometre thick. Much of the Ordovician rocks are now missing from the Shropshire sequences, but may have been deposited only for further erosion to take them. Again, this is seen as an angular unconformity in the area, where the Hoar Edge Grit returns deposits of late Ordovician age with a conglomerate. Some grains of sand in this formation have wind blown surfaces, which attest to arid desert conditions, again proving subaerial weathering, and include rounded, facetted pebbles.

Eventually, a shelf sea covered the land in dirtier, muddier waters, where the Harnage Shales formed, and this allowed a faunal province of trilobites, graptolites and brachiopods to develop. This was in low energy waters, once high-energy conditions returned the Chatwall Flags and Sandstones and then the Cheney Longville Flags were deposited. Fossils in these are mainly brachiopods and are preserved in lenses of shell banks within those sandstones. The shell banks are called the alternata Limestones after the vast numbers of brachiopods found in them. Toward the end of Ordovician times, quieter conditions again stabilised the area with the deposition of the Acton Scott Group and the Onny Shales, both of which have shelly lenses also.

Tectonics again changed things with movements along the fault system, so that here the latest Ordovician formations are not seen. The subduction in the NW caused massive movements that moved blocks along the fault zones to the NE. This lateral movement gave a NE/SW axis to folding and a correspondingly high amount of compressional forces were involved. Again, to the west, igneous intrusions were added. At the Wrekin, to the east, a lamprophyre sill, large enough to be economically quarried forced its way into the Shineton Shales. The SS are near vertical beds of green-grey sandstones with the dendroid graptolite Dictyonema. At the contact with the intrusion, baked margins to the shales prove their formation prior to it.

An angular unconformity to the end Ordovician, through the early Silurian, saw the area above sea level again. This further erosion, coupled with the raising of sea levels due to polar ice melt, inundated the land once again. In the area of the Onny Valley, this is seen as a 5^o difference in the junction of the Ordovician with the Silurian, where the Purple (Hughley) Shales rest unconformably on the Onny Shales. This is particularly difficult to see here in the valley as shales lie upon shales and there is only that small difference in dip. The actual dip of the Onny Shales is about 25^o; the Purple Shales lie slightly flatter at 20^o, to the SE.

Along the Onny Valley Trail several late Ordovician formations can be seen. The trail is along an old BR branch line and from the car park at the Craven Arms end, the trail follows close to the Onny River. The first exposure is on a bend in the river where the Ordovician/Silurian boundary is seen (see previous paragraph). Unfortunately grass and woodland cover much of the sequence. However, after crossing a number of styles, exposures of the Chatwall formations can be seen at the top of a cutting. About 100 metres on exposures of alternata Limestone and another 50 metres or so from there other exposures of the Cheney Longville Flags were seen. These also had fossils of Tentaculites along with several types of brachiopod in them. To complete the section the Hoar Edge Grit is reached, the beds of which dip 70^o SE, where they are in angular contact with Pre-Cambrian Longmyndian rocks.

As far as younger deposits are concerned they are not covered for this report, though they may be at a later date. All rocks in the area have been affected by more recent events i.e. Ice Age glaciation. During these times hilltops stood out above the ice termed 'nunataks', an Eskimo word. Once the ice fields had receded, melt water eroded and inscised faults and joints to form valleys such as Cardingmill and Batch. These cut into the eastern face of the Long Mynd. Glacial till covers the bulk of the Church Stretton valley of Silurian rocks, with more recent soil over that. On hills like Caer Caradoc and The Lawley, rocky tors stick out like 'knuckles' through a thin covering of head deposits.

This concludes my look at the geology of this part of Shropshire. The whole area is topographically attractive, in my opinion, with fields and woods, wildlife and farming, rivers and falls, a 'geological masterpiece'.

References:

Chidlaw, N., Unpublished Field Papers, 2003.

Toghill, P., Geology In Shropshire, 1990.

Toghill, P. and Beale, S., Ercall Quarries GA Guide No.48, 1994.

Earp, J.R. and Hains, B.A., The Welsh Borderland British Regional Geology 3^rd Edition, 1971.

Windley, B.F., The Evolving Continents 3^rd Edition, 1995.

Palmer, D., The Atlas of the Prehistoric World, Revised Edition, 2000.

BGS Church Stretton Sheet No.166, Solid, 1980.

Text by Dave Talbot

This is only an interpretation of the history of the geology in the areas mentioned, along with descriptions of the chronology, as I understand them. For this reason some details could be incorrect. As it is not my intention to mislead, anyone wishing to visit the area would do well to form his or her own opinions from the above references.

back