GEOLOGY AND SOUTH CERNEY
by Joe McCall, 44 Robert Franklin Way, South Cerney

 

INTRODUCTION

Surely, the geology of a small area such as a village cannot be of much significance? Yet, the Geology upon which this village rests determines its whole character, the fabric and appearance of its buildings and its main economic occupations - agriculture and sand and gravel extraction with its secondary industries of reconstituted stone fabrication. Geology is in fact fundamental to all land use and the character of the countryside, its villages and towns.

The Cotswolds as a region made their first impact on the National Economy on account of their limestone uplands, which, cleared of forest to rich limestone-fed grassland, were the basis for the mediaeval sheep industry: and the native limestone built the attractive fawn, honey-coloured and even brown buildings of the towns and villages. In those days they built using what material was available nearby and could be carted in - if stone was available they built of stone, if no stone they built of brick manufactured from clays (brick earths) or wattle and daub. These alternative materials do not appear to have been utilised in our village.

At the present time, the buried limestone rock provides much of the water for Cirencester and Swindon, and springs emanating from it feed the Churn River which flows through our village.

THE GEOLOGY

Discussion of the Geology may be subdivided into two sections:

1) The Jurassic Bedrock

2) The Superficial Deposits

The Jurassic Bedrock

The geological history of the Earth is immensely long - radiometric dating of isotope decay in minerals has shown that the oldest known rocks exposed at the surface in Canada are 4000 million years old (we have moved a long way since in 1654 Archbishop Ussher of Armagh determined the age of Creation at 4004BC!), but we are concerned here only with the Jurassic System, named after the Jura Mountains on the Franco-Swiss boundary, though in England the rocks of this System were first mapped by the 'Father of Geology' William Smith, a canal engineer, who was born at Churchill, near Chipping Norton, in the latter part of the 18th Century. These Jurassic Rocks have an age of deposition ranging from 205-135 million years ago (Table 1), and were deposited in ancient shallow seas - as soft sediments very slowly, were buried, compacted by the weight of burial (the material deposited above them), hardened, and were later returned to the surface by a combination of great earth movements which we call 'mountain building' and erosion by agents such as wind and rain acting on the Earth's surface to strip off the rocks later deposited above them. Our Jurassic rocks in the Cotswolds were folded and faulted only weakly in the outlying weak ripples of the mountain building that produced intense contortions and uplift of the rocks of the Jura and Alps in France and Switzerland, and are thus more or less evenly bedded, dipping generally eastwards and southwards off the high Cotswolds.

These Jurassic rocks form part of the north limb of an immense structural trough or Syncline. the centre of which is the London Basin (Figure 1).

Figure 1. The Geology of the Thames Valley showing the position of South Cerney over the Jurassic Rocks of the northern hinge of the great structural trough ('syncline') that forms the London Basin (from "London and the Thames Valley" by M.G.Sumbler, 4th ed. 1998. British Geological Survey Regional Guide, HMSO).

 

Table 1. The geological column for the last 250 million years showing the position of the Jurassic bedrock and periglacial gravel terraces.

There are numerous named rock formations in the Jurassic of the Cotswolds and our village lies above the Cornbrash (the latter so named because the early geologists thought it looked like the mass of broken corn fragments after threshing). This formation is dominated by limestone, but did not supply the principal limestone building stones of the Cotswolds. The best building stone, such as the Taynton Stone from near Burford, which was taken by canal to London to build St. Pauls, comes from the Inferior and Great Oolite formations below the Cornbrash, so called because they are composed of minute ovoid particles - ooids: chemically deposited in layers around little bits of sea shell (you can see ooids being formed in the shallow, warm seas off the Bahamas at the present time). The oolite makes excellent freestone, that is it can be cut and tooled into squared blocks easily and it also hardens on exposure to the weather. The two diagrams (Figures 2,3) below show the relationship of the oolite to the Cornbrash, which lies above them. The Cornbrash is at the top of the Great Oolite Group which. together with the Inferior Oolite Group below covers the age range of 163-178 million years ago.

Figure 2. Geological map of the area around South Cerney. The Jurassic Formations, coloured orange, green and blue are the bedrock formations: the Pleistocene terraces coloured buff comprise the superficial cover (after Institute of Geological Sciences, 1 inch series, Sheet 252, Swindon)

Figure 3 Cross section of the bedrock geology, Kemble to South Cerney Airfield (source, as above).

We can see the use of the oolite as bulding stone in our village as much of it is used in the interior of our village church ; parts of the south front and porch are also built of cream to yellow coloured squared blocks of it (Figures 4 and 5). The very finely squared freestone blocks seen in Figure 4 are known as ashlar. There is also considerable use of it in the older buildings but much of these and the walls in the village are constructed of a coarser oolitic limestone from the Cornbrash which contains many small shell fragments. The stone is yellow coloured when fresh cut. but weathers to a greyish-brown, rather drab colour, which is noticeable in all the old buildings of villages such as Crudwell and South Cerney which overlie the Cornbrash: local stone was quarried for all but the purposes where fine stone was needed, in which case the builders transported in the fine oolitic limestones from farther afield (Figures 6,7,8).

 

Figure 4. Finely squared freestone blocks of oolitic limestone framing a surround of rougher cut but similar stone around a mediaeval stained glass window composed of broken fragments ('quarries').: the vestry, All Hallows Church, South Cerney.

Figure 5. Honey coloured mortared freestone walling of oolitic limestone. East wall of the south porch, All Hallows Church, South Cerney.

Figure 6. Drystone walling using squared blocks, This is a limestone containing much more small broken fossil shells than the oolite shown in Figures 4 and 5, and was probably quarried from the local Cornbrash. It has weathered to a drab colour which is typical of the Cornbrash. Church Lane, South Cerney.

Figure 7. A mediaeval house. The walls are built of roughly shaped, mostly flattened, mortared blocks of limestone. The drab colour of the weathering suggests that it is the local Cornbrash. Note that the house to the left has had its stone walling cleaned and shows the original colour when fresh quarried and the mortar clearly. These houses are roofed with Cotswold 'stone slates', flat slabs of oolitic limestone quarried in localities where there is a closely spaced natural parting in the rock. These are very heavy and thus have to be graded from small at the ridge to large at the eaves, as is well seen here. Station Road, South Cerney.

Figure 8. Drystone walling along the Thames Severn Canal, to the north of South Cerney village. The canal builders used rough hewn, flattened slabs of a quite coarse grained limestone crammed with fossil shell fragments, from the local Cornbrash. It was probably excavated where, as here, they had to cut the canal through the bedrock. The wall has lately been carefully repaired with similar stone. which shows up bright yellow while fresh cut but will eventually tone down to the drab colour of the older wall stones.

You may well ask where are the outcrops of the bedrock and where are the quarries. There are remarkably few outcrops and remarkably few old quarries around South Cerney. A splendid section of the oolitic limestone has recently been exposed by the road cutting of the Cirencester By-Pass where it passes under the White Way, north of Cirencester. You can see the massive but overgrown outcrop of bedrock (Cornbrash) in old railway cutting immediately west of the northern road exit from the village to Preston Tollbar (Figure 9), but the bedrock is almost everywhere else buried by superficial deposits: the quarries must have become similarly obscured with the passage of time. The plateau on which South Cerney airfield is sited is composed of the Cornbrash. To the south and east of these outcrops, there are developments of clay formations, the main division of which is the Oxford Clay, above the hard limestone bedrock (Figure 9), as it dips gently downwards, but these are mostly obscured by the superficial gravels described below. These clays represent a transgression of the sea over the land after 157 million years ago, and they are fine deposits of a deepened sea.

Figure 9. Overgrown outcrop of massive limestone of the Cornbrash. Old Railway Cutting, west of the road leading north from Silver Street to Cirencester, South Cerney.

In the fields around the village you can pick up yellowish shell fossils shed from the limestone formations of the bedrock (Figure 10).

 

Figure 10. a) Bivalve Pholadomya deltoidea J. Sowerby

b) Bivalve. Possibly weathered specimen of the above.

Both found in fields near South Cerney, derived from the Jurassic limestone bedrock. Natural size.

 

The Superficial Deposits

The bedrock in, south and east of the village is overlain by the Upper Thames Gravels. These occur in four terraces, each originally spreading across the river valley but cut down successively by the river system, and so left fragmentally preserved on the sides of the valley (Figure 11). At South Cerney we have only the lowest and latest terrace (No.1 Terrace) preserved to any extent. These terraces occur over a wide area, south of Cirencester and Fairford, around Lechlade, and extend south into Wiltshire.

The gravel terraces are related to the Pleistocene glaciations, which occurred quite recently geologically speaking - these particular terraces range from about 300,000 to 10,000 years old. There are records of several glacial periods in the geological record and these are widely believed to be related to variations in the planet Earth's orbital eccentricities. The last glacial period which commenced just less than a million years ago had a number of glaciations within it, separated by interglacial periods when the ice retreated temporarily - we may be living now in such an interglacial period!

The gravel terraces were laid down when glaciers from the west, north and east converged on the midland plain and periodically advanced further southwards, During these later glaciations the ice did not surmount the Cotswold scarp and the terraces were formed in what we call 'periglacial' conditions, conditions just like those of the Russian or Canadian 'tundra', of treeless boggy expanses, south of the limits of the main ice sheet. The gravels are fairly well sorted and are composed overwhelmingly of fragments of the Jurassic oolitic limestone from the Cotswold bedrock (Figure 12). There are other rock fragments which have been derived from the drift deposited by the ice to the north. A feature of these gravels is the paucity of fossils - a search of the gravel banks southeast of the village by the writer has yielded a solitary bivalve (mussel) shell fossil (Figure 13)! However, in the gravel pits, near the base of the section, one may find fossils including ammonites (extinct cephalopods) (Figure 14) and corals (Figure 15), derived from the bedrock below, and Cleveland Farm Quarry south of Cerney Wick, in Wiltshire, has yielded important ammonite finds: a quarry south of Fairford has yielded corals. Cleveland Farm has also yielded a Mammoth tusk which is now in a museum at Bristol (Figure 16a). The mammoth, a relative of the elephant with coarse hair (Figure 16b), lived here more or less at the same time as the glaciations, and the tusk comes directly from the gravel. These gravel deposits were deposited in very strong flowing streams, seasonal torrents at times of thaw, coming off the ice as it melted - nothing like the present rather weak flow of the Churn and Thames - and such torrents could carry along a mammoth tusk, a metre or so in length. There is always the chance of finding a 'mammoth graveyard' in these gravels, like that recently discovered at Stanton Harcourt, near Oxford: or a mammoth abattoir, with bones accompanied by Palaeolithic tools, as has recently been uncovered just over into Wiltshire from Cerney Wick. The mammoth is widely believed to have become extinct on account of killing for food by primitive Man.

 

Figure 11. Sketch map of the distribution of the gravel terraces of the upper Thames Valley around South Cerney.

 

Figure 12. Gravel of the lowermost (first) terrace, north of South Cerney.

Figure 13. Small bivalve found in the gravels at the east end of the village. Natural size.

Figure 14. A Jurassic ammonite, Asteroceras. Natural size.

 

Figure 15. Jurassic Coral fossils found in terrace gravels, near Fairford.

Figure 16a Mammoth tusk recovered from a gravel quarry south of the village, in Ashton Keynes parish, in 1992. Picture   reproduced by courtesy of Wilts & Glos Standard.

Figure 16b A drawing of a hairy mammoth

These gravels supply the material for the aggregate industries of the neighbourhood and the sand and gravel materials provide the raw material for the locally manufactured 'Bradstone' (reconstituted stone) building blocks, of which many of the modern houses in the village are constructed (Figure 17).

The recreational lakes of the Cotswold Water Park which come right up to the edge of the village houses are the sites of former gravel workings. Because of the high water table hereabouts one cannot put landfill into these excavations as it invariably contains polluting materials and these would enter the Thames drainage system and contaminate water resources downstream. Therefore the excavations are left to fill with water and provide recreational lakes. So the lakes, though Man-made, are directly related to the geology, which has directed the radical change in the character of our village during the past forty years. Where there was a vista of flat green meadows stretching towards Cricklade and Ashton Keynes, there are now expanses of water carrying the white sails of yachts, and populated by swans, geese, mallard, coots and terns (Figure 18).

Figure 17. The wall of a modern house built of reconstituted stone, locally manufactured.

 

Figure 18. A recreational lake in the water park north of the village, a former gravel quarry. Gravel in the foreground.

THE GEOLOGY AND WATER

The geology controls the local ground and surface water systems. The rocks such as the two oolite bedrock divisions carry water in pores between mineral grains and in fractures, they are fed by percolation from rainfall. Besides this run-in, much of the rainfall runs off into surface streams such as the Churn. The streams are also fed by springs, especially in winter, after continued rainfall, water being fed back to the rivers from underground. There is a delicate balance between the water underground and the stream flow, and lately we have seen the flow of the Churn diminish in summer, the river bed even drying out. Large quantities of groundwater are pumped at Baunton and Latton to supply Cirencester and Swindon, and studies by the Environmental Agency have shown that this pumping does adversely affect the flow of the Churn. Also in its lower parts, below the village the flow is affected by evaporation from the artificial lakes, water from the Churn passing through the porous gravels to recharge them at the expense of the river flow. That the flow has diminished is evidenced by the fact that there were two mills in the mediaeval village - the present summer flow would have left them idle for months and clearly this did not occur. The main cause of the diminished flow of the Churn through the village is the pumping, but rectifying the situation is not an easy matter. Other than building reservoirs upstream, to conserve winter flow and utilise the water to maintain the flow in summer, the solution can only be in finding new supplies for the towns.

CONCLUSION

So we see that even in the context of a village, the geology has significantly influenced the character of the village, both in its mediaeval character and its modern changes: it has influenced the building materials used throughout its history, its industry and its water resources.