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Home , Little Avon River, Tortworth

Field Trip Report

The Tortworth Inlier Sunday 26th April 2015.

Dave Green, our group leader, showed us some of the fascinating geology, to the north-east of the city of Bristol. This area is relatively flat but has some undulating topography as it runs into the Vale of Berkeley. It includes the northern part of the once busy Bristol Coalfield. This part of South is largely rural, dotted with farms, small villages and little townships. It is enclosed by the Mendip Hills to the south, the Cotswold Escarpment to the north and to the east, and the Severn estuary to the west. The area is drained by small streams and like the Frome, whose waters skirt the contours and tumble into the Severn Estuary.

Palaeozoic rocks can be found exposed here, while to the east and north east younger Mesozoic rocks overlie them. The geology of at first blush appears complex and takes a little unravelling.

To begin with; some terminology

An inlier is an island of older rock strata, surrounded by younger strata. The Tortworth Inlier comprises Cambrian/Silurian outcrops surrounded by Mesozoic rocks located around Green, just south of the Little Avon . It also lies North East of the Carboniferous syncline once mined for the bituminous coals it contains.

The rock strata we hoped to see Cambrian rocks

The oldest strata are the Micklewood beds of upper Cambrian/Lower Ordivician and Tremadoc in age. These are grey micaceous shales with flaggy sandstone or siltstones inbetween. They lie in the southern third of the Tortworth inlier. The underlying and older Breadstone shales make up two thirds of the inlier. These are thin interbedded siltstones a few centimetres thick. Their fauna is typical of that found in the lower Tremadoc series.

An Ordovician unconformity

There is a notable absence of any later Ordovician rocks in this Palaeozoic succession. Yet to the north in Shropshire and Wales are found the graptolite bearing black shales that characterise much of the Ordovician.

Silurian

The younger Silurian rock strata are exposed in an orderly north west, south easterly direction. The first of these lies uncomformably upon the Cambrian rocks and are volcanic in origin. These are the so called Lower Trap. (Lower Trap meaning ‘lower step’ from the Sanskrit word for step). These rocks were at one time thought to be intrusive in origin but now considered to be extrusive and comprise of very altered amygdaloidal basalts. Above the Lower Trap are found the Damery Beds made up of thin bedded sandstone, siltstone and mudstone and the occasional layers of impure limestone. They range in colour from greys to greens to reds and purples. These beds contain fossils that are Silurian in age. These in turn are overlain by the Upper Trap a much altered micro-crystalline basalt with quartz xenocrysts visible under a hand lens. These quartz xenoliths distinguish this lava from the lava of the Lower Trap. Again this basalt is much altered. The rock weathers to a distinctive burnt sienna colour again with numerous voids. The voids are distorted gas vesicles indicative of degassing of a cooling, yet mobile magma that flowed and covered the land surface. The Upper Trap is overlain in turn by the Tortworth beds, similar lithologically to the Damery Beds but they are much less fossiliferous. At the base of the Tortworth beds is the Palaeocyclus band, a thin decalcified sandstone containing fossils of button corals. Above these are found the younger Brinkmarsh Beds. Wenlock in age, they are mainly grey mudstone with inter bedded thin bands of siltstone and fine grained sandstone. The Brinkmarsh beds are overlain by the Ludlow series with three distinctive mudstone beds.

Devonian

The Old Red Sandstone (ORS), the hallmark of the Devonian period, is present but the entire series is incomplete. In this locale, it is divided into the Lower and Upper Sandstone with a marked unconformity between them. They outcrop around the edge of the Coalpit Heath Syncline. Carboniferous

The Devonian ORS is inturn overlain by the Lower Carboniferous strata. The carboniferous limestone forms the most extensive outcrop of all Palaeozoic rock in the area. Today it is quarried extensively and is used for road stone and as a construction aggregate. In the past limestone was also heavily quarried for use locally. Carboniferous rocks also include quartzitic sandstones and the coal measures. The Pennant Sandstones were used locally for building and dry stone walling. Coal deposits were mined in several localities and coal mining peaked in the 1870-90s. Mining now has ceased due to the exhaustion of workable seams and increasing difficulties of the geological conditions.

Permo-Triassic

Rock interface marks a major unconformity.

Upper Triassic

The Triassic rocks are mostly sandstones, mudstones and conglomerates which lie unconformably on deeply folded Permian beds but in this area they lie directly on top of Carboniferous strata.

An enthusiastic group, gathering at the start of the trip. A joyous Heidi, pleased that the sun is shining!

The Field Trip

1. Tytherington Hill Quarry, Scarp and Coalpit Heath Syncline.

To begin we all met at Tytherington Hill ST672886. We parked at the side of Baden Hill road and as a group we proceeded to examine the exposed surface of the western wall of an abandoned limestone quarry. The angles of dip on the exposed quarry slab face are steep, dipping at some 55 degrees towards the East. This is part of the western arm of the syncline of Carboniferous limestone rocks. Carboniferous fossil remains of broken crinoids, bits of corals and broken shell fragments (brachiopods) were encased in a hard limestone matrix. The shell detritus indicated deposition had taken place within a shallow mobile marine environment subject to the action of waves.

An exposure of crinoids in carboniferous limestone

We then climbed to the top of the scarp of this old quarry by a steep flight of steps to the left of the quarry. The scarp runs in a NNW direction with the road running below it. Facing East we could see a series of low ridges running parallel to the ridge line we were standing on. These are the more durable outcrops of Carboniferous limestone rock that mark out the Western limb of the Coalpit Heath syncline. The syncline continues over the horizon and we would visit the Eastern part of the syncline later in the day. We were to find that the Eastern parts of the syncline had much shallower angles of dip. The Western limestone ridge on which we were standing runs from though to Thornbury and onwards in a NNE direction. Whilst on the Eastern side the eastern Limestone ridge runs from to village in a NNW direction. At the northern end these limestone ridges mark out a horse shoe shaped perimeter of the basin.

The limestone in this area has been quarried extensively and continues to be worked today by Hanson, part of the Heidelberg cement group. In the past Hotwells limestone was quarried locally for building the grey limestone walls used to enclose fields and to build houses. It was also used for road making and burnt in lime kilns to make quick lime (Calcium oxide), an ingredient of lime mortar. Quick lime when mixed with water reacts exothermically to make slaked lime. Slaked lime was used to whitewash walls and can also be used to adjust the pH of acidic soils. The divalent Calcium cation in lime has the ability to cross link negatively charged clay particles and to improve the crumb structure of the clinging clays of many local fields.

2. Brinkmarsh Quarry

Our next port of call was to Brinkmarsh Quarry. We parked in a lane near the A38 and walked to the quarry at ST675914. The quarry is edged by a line of trees and is hidden from the road by a low hill. In the quarry the Wenlock shales are overlain by limestone beds as part of an small anticline. The limestone has been quarried and removed leaving a muddy reddish brown clay bottom that in wet weather is heavy enough to impede walking.

The intrepid group, traversing a muddy field towards the Brinkmarsh quarry.

An exposure of crinoids in carboniferous limestone. The Brinkmarsh shale beds are exposed near to the entrance to the quarry. The eagle eyed observers in our party soon began to find fossils. The Silurian rugose coral Pycnastis was abundant and rapidly we had found several samples as well as the fossil remains of several small bivalves.

Fossils found in the pycnactis band. A collection of bivalves and Silurian rugose corals!

Dave Green pointed out a nodule of Celestine lying above the flaking impervious shales. Celestine is a mineral of Strontium Sulphate found in , and Clifton. The concentration of the element Strontium is low in sea water today but can be much higher in hydrothermal deposits. Locally Celestine is associated with the Mercia Mudstones, which are Triassic in age. Celestine is thought to have been deposited along with other paragenic evaporites like gypsum (Calcium Sulfate) and halite (Sodium Chloride), in land locked seas or lakes. The original concentration of Strontium may have taken place in the aragonite shells of molluscs. Aragonite can contain 8000ppm of Strontium as opposed to calcite which contains only 400ppm. Over time the aragonite changes to its more stable polymorph form, calcite. The change releases Strontium metal ions in the process. The Strontium metal ion is more reactive than the lighter alkaline earth (Group 2) metals Beryllium, Magnesium and Calcium, and which in aqueous conditions Strontium is able to displace them to form Strontium salts. Strontium ions in turn can be displaced by more reactive Barium ions when these are present. Once released the heavy Strontium ions are able to travel down to lower strata as they continue to be leached out from the source. The relatively rare Strontium ion, can be concentrated further by the displacement of Calcium contained in insoluble minerals such as gypsum or anhydrite. Nodules of Strontium sulfate build up as the insoluble mineral Celestine when leached salt solutions are confined by impermeable clays and shales such as the Mercia mudstones and the Silurian Brinkmarsh Beds. Celestine has an orthorhombic crystal form. Its colours vary from white, to yellow, to brown, to pink, to the pale sky blue of the seminal sample after which the mineral was named. The colours depend upon the presence of impurities within the crystalline matrix. Celestine is also known as celestite or locally as ‘spar’. It is insoluble in water and has a high melting point (1600 degrees Celsius) typical of an ionic compound. It is relatively soft, with a value of 3-3.5 on the Mohs scale of hardness. It is 4 times as heavy as water with a specific gravity of about 3.96 which helps to distinguish it from nodules of calcite, gypsum or anhydrite in the field. Celestine was mined extensively in the area and sold abroad and at one time was responsible for 90% of the world production of this valuable mineral.

Hand sample of Celestine. Celestine is the principal source of the element strontium, and can be used in fireworks and in various metal alloys

We continued up towards the end of the quarry where the durable mudstone was overlain by a limestone bed. Here there was evidence of faulting and in the far corner of the quarry a vertical fault marked by fault margin breccia runs vertically down into the rocks below. The more durable limestone rocks showed evidence of cross bedding indicative of deposition in a shallow marine environment.

The Berkeley and Faults

Corner of the quarry, showing cross bedding and a vertical fault line.

Dave Green, ably assisted by Heidi, giving an explanation of horst and graben features, in particular, discussing the Berkeley fault. The graben are the downdropped blocks and the horst are the upthrown blocks that lie next to the graben.

Upon leaving the quarry we held a discussion about the nature of the faulting that ran along the anticline and the Berkeley and Iron Acton faults in particular. The geological map we were using showed the area has several faults running in a South North direction. It is considered that the compressive forces generated between Baltica and Laurentia as they came together, produced an East-West squeeze in this area and led to a series of elevated Horst features. Once raised they were eroded. The Berkeley fault has a thrust of some 1000m and the Iron Acton fault a thrust of some 500m but also both faults have a horizontal component to their displacement making them both thrust slip fault formation.

The fault disappears below mesozoic strata to the north but is in direct line with the Malvern Horst Thrust slip fault system. It is tempting to make that leap of faith and consider these faults to be part of a Malvern Fault Belt. There are also examples of Klippen in the Mendip Hills to the South.

3. Brook Farm Quarry

The next stop was at the Brook Farm Quarry ST721928. This had an outcrop of Old Red Sandstone which we could examine. The upper ORS was deep brown in colour due to the oxidised iron and full of quartz fragment sand. It also showed some bedding, indicative of deposition under desert conditions. These outcrops marked the western edge of the syncline.

4. A light lunch at the Buthay Inn in and a local link with the Battle of Crecy.

The area around Wickwar was called Wychen when the Doomsday book was being prepared. The name changed after King John granted lands to John La Warr during his reign. Wychen became the ‘wick’ or the village of the Warr’s, and ever since Wickwar. During medieval times it was compulsory for yeomen to practice archery. Fields containing earth mounds as targets were used for this purpose and were called archery butts. The Buthay Inn is reputably situated on such a site. This explains the unusual name and why the inn sign sports an archer. There is even a picture of medieval archers in the restaurant! It turned out that another John La Warr fought alongside the Black Prince at the Battle of Crecy where the flower of French nobility were cut down by the armour piecing arrows shot from English longbows. After a light lunch we got in our cars and made our way to Bury Hill quarry.

5. Bury Hill Quarry

There are a number of Bury Hill sites in this area. This site ST723856, is off a public footpath. We parked near Farm at ST723856 and proceeded to walk down the road to the footpath. We followed the footpath uphill across a field towards a wooded ridge. Entry to the quarry involves a steep decent which we all negotiated safely by using a zig zag traverse between strategically positioned trees.

Successfully negotiating a zig-zag path through the trees!

This quarry shows the much eroded pale grey limestone belonging to the Hotwells Limestone bed of the Lower Carboniferous which we encountered at the first quarry on the western limb of the syncline earlier today. It is overlain by a bed belonging to the Dolomitic Conglomerate of the Mercia Mudstone Group. The Hotwells limestone dips into the quarry wall at about 30 degrees. Between the beds lies a major unconformity with a gap in deposition of some 100 Million years.

The top of the Limestone is marked by a dipping undulating wavy eroded platform with a repeating wave length of 15-20 metres. Could these be the features of the erosive action of a long lost Triassic sea?

This limestone is fine grained and belongs to the Hotwells Limestone bed. It is hard, with crinoids and other shelly fossils and described as being intraclast in appearance. When struck with a geological hammer it produces a distinctive ‘metallic’ like ring indicative of a sedimentary rock that to all has undergone considerable compaction and change.

Donning a hard hat, Pauline pointing out the dipping limestone and then showing of a Major unconformity visible

Another view of the spectacular wavy pattern of the Hand specimen of Dolomitic Conglomerate perhaps unconformity. Triassic beds are on top. though more a breccia?

The UpperTriassic Dolomitic Conglomerate on the other hand is a scree/debris flow of a poorly sorted angular breccia containing angular limestone clasts 2-5 cm across, angular feldspars 2-3cms, angular 1mm quartz fragments and larger fragments of crystalline quartz in a buff matrix of sand and mudstone with occasional shale clasts.

Such a mix of fragments suggests weathering and erosion under desert conditions. Rocks would be carried by ephemeral flash floods along wadis and deposited onto inland deltas. The Dolomitic Conglomerate name for this bed really could do with a change, as the rock has more in common with a breccia than a conglomerate, with little abrasion or sorting due to the action of water, at least in the samples we examined. One similar source of Dolomitic Conglomerate rocks can be traced as originating from the Mendips.

This unconformity left us wanting to learn more but we had to leave as time was running on. We made a fast exit out out of the quarry by taking an interesting scramble up and out through a hedge into the field above.

We now proceeded to our next destination, Cullimore’s Quarry in Chartfield.

6. Cullimore’s Quarry

We Parked along a lane at ST721928 continued and followed a footpath that crossed the Birmingham to Bristol railway line and arrived at a field with a worked topography. Protected by a hedge lay a small quarry that had been cleared but had been recolonised by woody saplings and brambles.

From the map we can see that the orientation of the strata in the Tortworth inlier point to a major pre Variscan lineament.

The source of the Silurian traps remains hidden and probably lies under Mesozoic strata and is based on the 300m thicknesses of Silurian Traps lavas obtained from bore hole data drilled further to the east. It seems that the Silurian Traps beds exposed in the Tortworth Inlier thinned as they spread out. The basalt lavas are Llandovery in age, when they crystallised and cooled towards the limit of the flow. The Upper Trap is a microporphyritic basalt containing xenocrysts of quartz that sparkle in the light and that are clearly visible under a hand lens. There are no pillow lava structures that would indicate marine deposition and the presence of trapped twisted gas voids indicate degassing in a hot flowing yet rapidly air cooled lava flow. The basalts are much altered and ruddy in colour.

Cullimores Quarry Exposure being rapidly overgrown by brambles and shubs.

The lower contact of the Silurian Trap is not visible in this quarry. The Silurian Trap is overlain by fine grained sandstones with clay bands of the Tortworth beds and which are of a Llandovery age. These beds contain marine fossils and indicate that prior to the eruption there may have been some uplift and this was once again followed by a return to marine deposition.

Tectonics and the Tortworth Inlier

The big picture. Our packed introduction to this area focused in the main on the Palaeozoic era.

The youngest sedimentary rocks were being laid down at a time when Baltica began to be subducted under the northern part of the Avalonianisland arc some 510 million years ago. Tectonic movements began to close the northern arm of the Iapetus Ocean called the Torquist Sea. Baltica continued to close with Laurentia well to the north and subjecting strata to an East West squeeze. Avelonia, an island arc micro continent , had previously rifted from Gondwana, now began to be squeezed into these northern land masses.

The buckling and compression could be responsible for the volcanic activities during Silurian times and could explain the reduction in fossils numbers found in marine strata before and after these events. Avelonia continued to migrate northwards on its’ journey across the equator. These tectonic events left an indelible mark on the geology of this area.

The closure of the Iapetus Ocean enabled a part of the island arc of Avalonia to be squeezed out as it began to suture with Laurentia. Towards the north, the marine deposits of the Iapetus Ocean were folded, raised and rifted and then eroded to expose the geology we saw today.

Gondwana’s collision with this northern complex itself took tens of milions of years. It is thought that as the WNW-ESE trending Variscan Deformation Front propagated northwards. During the Westphalian period of the Carboniferous a fore deep was produced. As the lithosphere flexed and buckled it provided accommodation space in the southern regions of Britain for marine deposits to settle. It was into this subsiding paralic basin that the Westphalian peat mires of southern Britain produced the Carboniferous coal deposite of the North Bristol Coalfield.

To the south west of the Avelonian archipelago, the back arc basin that made up the Rheic Ocean was eventually squeezed out and closed. The Appalacian-Mauretanide-Variscan (AMV) orogenies formed the Central Mountains of the supercontinent Pangaea. This continent was to last some 100 million years. Continent on continent collisions produce mountains the like of the Himalaya. Collectively the AMV orogenies mountain ranges combined to formed the Central Pangaea Mountain Range. They stretched for thousands of kilometres to the East and West. They could have reached to hights of over 1500m. Above the snow line, these snow-capped peaks fed valley glaciers that began the processes of their weathering and led to their levelling. The Mendip Hills were close to the northern limit of this orogeny and they subjected the depositional basins to considerable pressures. The foreland basins were deformed into a series of anticlines and synclines. The movements propagated from south to north and so the rocks became folded. The raised rocks became subjected to erosion. The contact of Pangaea with Avalonia was oblique starting from the south and continuing to the north. A series of thrust and shear faults trending from south to north relieved the stresses and resulted in the Horst and Grabben like formations in this area. The mountain chain that once ran in a East West direction has long gone. It’s remains have left us the Mendips, the Quantocks and Exmoor Hills further to the west. The Carboniferous strata buckled during the Permian and once cracked became rapidly worn and eroded. Where the land was raised the underlying rocks would later become exposed and in turn be subject to erosion. The climate of Pangaea became hot and dry during the Permian. Desert conditions weathered shattered and eroded the mountains and hills. Prevailing winds would have winnowed the sands and dusts away. Rocks weathered under desert conditions produce angular fragments. Such fragments were deposited by flash floods or broken down further into fine sands. Fine sands could have been deposited as beds of loess.Hot mantle plumes rising below Pangaea melted the lithosphere and rifted the Pangaean supercontinent.

Pangaea began to split apart. In this region of South Gloucestershire there are no strata belonging to the Permian period. They have been eroded away. The end of the Permian was also accompanied by a mass extinction, the so called Great Dying, when only 5% of the organisms survived. During this period the Prototethys Ocean began to form and to rift and grow marking a return to a period of deposition under marine conditions.

The next group off strata belong to the Triassic period but not the distinctive Triassic three layers that mark this period of deposition in Germany. In Germany the Triassic sandstone beds are overlain by a limestone and they in turn are overlain by shale. Needless to say that due to uplift, erosion and faulting all of this sequence is missing in this area. The beginning of the Mesozoic era is marked by a major unconformity with the Palaeozoic. It is between the Hotwell Limestones of the Lower Carboniferous and the Dolomitic Conglomerate of the upper Triassic. An enormoustime gap of more than 80 million years.

We had covered a good deal of Palaeozoic geology on this day in what was in effect a blink of the eye. The company was great and as is often the case we were left with more questions to consider. We thanked Dave Green for his time and guidance and left for home.This area definitely warrants a return visit.

Expression of thanks by the group for Dave Green.

All photographs taken on the day by Don Cameron.

Bibliography

Arkell, J. (1935) ‘Analysis of the Mesozoic and Cainozoic folding in ’, Report of XVI International Geological Congress Washington, 1933.

Green, D. (2015) private communication handouts.

(i)Table of strata in the Wickwar-Hawksbury area.

(ii)Map of the Silurian rocks of the Tortworth Inlier Gloucesterhire.

(iii)Geology and scenery around Cromhall with a cross section through the Coalpit Heath Syncline.

Green, G. W. (1992) British Regional Geology: Bristol & Region (British Regional Geology), 3rd ed. London, Her Majesty’s Stationery Office.

Kellaway, G. A. and Survey, B. G. (1993) Geology of the Bristol district. By G.A. Kellaway and F.B.A.

Welch, London, Her Majesty’s Stationery office.

Lowe, A. (n.d.) ‘Celestine | SGMRG - South Gloucestershire Mines Research Group’, [online] Available from: http://www.sgmrg.co.uk/celestine (Accessed 2 July 2015).

Nance, D. R., Gutiérrez-Alonso, G., Keppie, D. J., Linnemann, U., Murphy, B. J., Quesada, C., Strachan, R. A. and Woodcock, N. H. (2012) ‘A brief history of the Rheic Ocean’, Geoscience Frontiers, 3(2), pp.125– 135.

Stamper, C. (2012) ‘Minerals of the Avon Region Celestine (SrSO4)’, OUTCROP, [online] Available from: http://avonrigsoutcrop.blogspot.co.uk/2012/06/celestine-srso4.html (Accessed 1 July 2015).

WILLIAMS, G. D. and CHAPMAN, T. J. (1986) ‘The Bristol-Mendip foreland thrust belt’, Journal of the Geological Society, 143(1), pp. 63–73.

Wallis, R. (2014) Yate, GCHT.

Welch, F. B. A. (1968) Bristol and Gloucester district, 2nd ed. London, HMSO for Institute of Geological Science.

Zbig Towalski and Pauline Kirtley.