Highlights of Jurassic Warwickshire

© Blackwell Publishing Ltd, Geology Today, Vol. 21, No. 4, July�August 2005146

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The central English county of Warwickshire (Fig. 1)has a diverse geology that ranges in age from latePrecambrian to Quaternary. Central and northernparts of the county, including the WarwickshireCoalfield, are dominated by a thick succession ofUpper Carboniferous up to Triassic red-beds. Mucholder rocks � Precambrian volcanics, Cambriansandstones and shales, and Caledonian diorite andlamprophyre intrusions, occupy the northernmargins of the coalfield. In contrast, the landscape ofsouthern and eastern Warwickshire is dominated bya clay lowland known as the Feldon, fringed to thesouth by escarpments and outlying hills of theCotswold fringe. The Avon Valley runs along thenorth-western edge of the Feldon and stretches awaytowards the Worcestershire border. These areasconstitute Warwickshire�s picturesque �ShakespeareCountry�. They are underlain by several hundredmetres of richly fossiliferous marine Lower andMiddle Jurassic strata (Figs 2 & 3), showing a gentleregional dip towards the south-east.

During the 19th and early 20th centuries, theJurassic succession was exposed in many quarriesand cuttings, attracting the attention of geologistsincluding Peter Brodie, Horace Woodward, EdwinWalford and Linsdall Richardson. Exposure is nowlimited to a handful of working and recentlyabandoned quarries, largely overgrown road andrailway cuttings, and numerous small scrapes instreams, ditches and shallow, weathered landslips.Several sites, mainly disused quarries, are protectedas Sites of Special Scientific Interest (SSSIs) andRegionally Important Geological Sites (RIGS).Additionally, museum and British Geological Surveycollections provide a valuable resource, allowingmodern investigation of rocks and fossils from long-lost sites. Local stone has been extensively used forbuilding purposes in many villages on the Jurassicoutcrop. The attractive rust-coloured ironstone of theMarlstone Rock Formation is particularly widespreadand easily recognized.

FeatureHighlights of Jurassic WarwickshireSouthern and eastern parts of the central English county of Warwickshire are underlain by a poorlyexposed but well-preserved and richly fossiliferous succession of Lower and Middle Jurassic strata. Thisarticle outlines the nature of the Jurassic sediments and their palaeoenvironments, and highlights someof the more important sites and discoveries.

Jonathan D.RadleyWarwickshire Museum,

Market Place, Warwick,

Warwickshire, CV34 4SA,

UK.

[email protected]

Geological background

The north-south trending Vale of Moreton Axis runsthrough south-western Warwickshire (Fig. 2) and isone of several deep-seated major structures thatclearly affected early Jurassic deposition in southernBritain. Their existence has been known togenerations of researchers, and gave rise to the well-known concepts of �axes of uplift� and �basins andswells�, as major controls on Jurassic faciesdevelopments, cyclicity and sediment thicknesspatterns. The Vale of Moreton Axis region broadlycoincides with a number of changes in the

Fig. 1. Map showing locationof Warwickshire, centralEngland.

VA

Stratford-upon-Avon

Warwick Leamington Spa

Shipstonon Stour

Rugby

VA Vale of Moreton Axis

Lower Jurassic outcrop

Nuneaton

Middle Jurassic outcrop

Edge

Hill

N

10km

Fig. 2. Outline map of Warwickshire showing the Jurassic outcrop.

© Blackwell Publishing Ltd, Geology Today, Vol. 21, No. 4, July�August 2005 147

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thicknesses and sedimentary characters of parts ofthe succession, suggesting the former presence of anactive structural �hinge� in Early and Middle Jurassictimes. The region to the west of the axis marks theeastern part of the Worcester Basin, where theGloucestershire Cotswolds and Severn Valley areunderlain by a thick Lower-Middle Jurassicsuccession. To the east, in south-easternWarwickshire, the sediments were deposited near thesouth-western end of the East Midlands Shelf; a stablearea characterised by relatively low rates ofsubsidence during the Jurassic Period. The LondonPlatform, with its emergent core, was situated to thesouth east.

It is generally agreed that much of the onshoreBritish Jurassic was deposited in shallow water,normally no more than a few tens of metres deep.The presence of fossil corals, rich molluscan faunas,oolitic limestones and ironstones shows that climates

were warmer than those experienced by Great Britaintoday. The Lower and Middle Jurassic ofWarwickshire consists of two overall upward-shallowing marine successions, deposited underregional tectonic control, superimposed upon ascenario of general Jurassic sea-level rise. Making upthe bulk of the Jurassic succession, the deepest-waterphases are represented by mudstone-dominatedformations. Examples include the Lower Jurassic BlueLias, Charmouth and Whitby Mudstone formations(Fig. 3). Such units are commonly rich inbiostratigraphically important ammonites, althoughbenthic faunas dominated by bivalves, brachiopodsand echinoderms are also widespread. Themudstones pass up into thinner successions ofsiltstone, sandstone, limestone or ironstone,containing fossils and sedimentary structures ofshallower-water aspect. The first upward-shallowingepisode culminated in the oolitic ironstone of thePliensbachian, possibly up to Toarcian (LowerJurassic) Marlstone Rock Formation; the second withBajocian-Bathonian (Middle Jurassic) limestones andsandstones of the Inferior and Great Oolite. TheBritish Geological Survey developed the currentlyused lithostratigraphical scheme (Fig. 3), outlined intheir most recent memoirs, sheet descriptions,research reports and published papers. The ammonitebiostratigraphy was summarized in the GeologicalSociety of London�s correlation charts for the Lowerand Middle Jurassic, published in 1980.

Lower Jurassic

The Wilmcote Limestone Member (basal Blue LiasFormation) ranges in age from latest Triassic(Rhaetian) up to early Jurassic (Hettangian) and is upto about 10 metres thick. The Wilmcote limestoneswere once quarried as a source of building, wallingand paving stone and agricultural lime in the AvonValley, and are conspicuous in the older buildings ofvillages such as Wilmcote and Temple Grafton. Sitessuch as Wilmcote Quarry still expose a few metres ofmudstone interbedded with fine-grained laminatedlimestone, deposited largely as poorly oxygenatedmuds during the earliest phase of the Jurassic marinetransgression. When in work, the quarries yieldedbeautifully preserved fossils including crustaceans,insects, fishes, ichthyosaurs, plesiosaurs and landplants (Fig. 4).

The Wilmcote Limestone outcrops west of the Valeof Moreton Axis. On the shelf area to the east, theearliest Jurassic sediments are represented by theSaltford Shale Member of the Blue Lias Formation.Eastern and south-eastern Warwickshire has longbeen associated with the Rugby Cement industry.There, the Blue Lias strata were formerly exposed in a

Fig. 3. Outline stratigraphy ofthe Jurassic succession inWarwickshire.


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number of large quarries that yielded manyimportant fossils (Fig. 5). This geological andindustrial heritage is celebrated for example by anumber of circular walks around the village ofStockton known as the �Blue Lias rings�, an image ofan ichthyosaur skeleton on a road traffic sign atStockton (Fig. 6), and the Blue Lias public house nearLong Itchington.

Southam Cement Works Quarry, near LongItchington, still exposes the Saltford Shale Memberand lower part of the overlying Rugby LimestoneMember of the Blue Lias Formation, above Rhaetian(Langport Member) limestones (Fig. 7). This sitecurrently provides Warwickshire�s most extensiveexposure of Jurassic rocks. The Saltford Shale isroughly 15 metres thick and ranges from the LiasicusBiozone up into the Angulata Biozone of theHettangian Stage. It is dominated by grey laminatedmudstones enclosing nodules and thin beds of fine-grained limestone. Some of the nodules are packedwith well-preserved schlotheimiid ammonites(Fig. 8), as well as fish debris, bivalves, nautiloids andreptile remains. The ammonites are commonlyimbricated, suggesting that some or all of the skeletaldebris accumulated under the influence of weakcurrents. The nodules are the principle source ofSaltford Shale fossils at the Southam quarry.Warwickshire Museum recently acquired an

Fig. 4. Ichthyosaur skullcollected from the WilmcoteLimestone Member (Blue LiasFormation) at Binton, west ofStratford-upon-Avon, during thenineteenth century.Warwickshire Museumcollection. Specimen is 45 cmlong.

Fig. 5. Ichthyosaur skeleton atHarbury Quarry, 1928.

Fig. 6. Speed limit sign,Stockton.

Fig. 7. Southam Cement Works quarry. Excavations in the foreground expose the late Triassic (Rhaetian)Langport Member, about 2.5 metres thick. In the background, the main face is roughly 35 m high. Itexposes the early Jurassic Saltford Shale Member overlain by alternating mudstones and limestones of theRugby Limestone Member. The Saltford Shale and Rugby Limestone belong to the Blue Lias Formation.


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extensive collection of fossiliferous nodules from thissite, including partial ichthyosaur and plesiosaurskeletons.

As in the Wilmcote Limestone, the abundantlamination and scarcity of benthic fossils in theSaltford Shale suggests an overall poorly oxygenatedsetting. However, cores from the British GeologicalSurvey�s nearby Stockton Locks and Harbury Quarryboreholes revealed sedimentary cycles of fissilemudstones passing up into relatively massive,calcareous units. These suggest that the sea floorbecame periodically oxygenated. Above the SaltfordShale, the lowest part of the Rugby LimestoneMember is a typical �Blue Lias� development of rapidlyalternating mudstones and fine-grained limestones,forming the upper part of the Southam section(Fig. 7). Unlike the Saltford Shale, these rocks arecommonly riddled with burrows and contain manybenthic fossils including giant bivalves (Plagiostoma),pleurotomariid gastropods, rhynchonellidbrachiopods, echinoids, crinoids and rare corals. Thepalaeontological evidence suggests a relativeshallowing of the Lias sea during the late Hettangian.

Above the Rugby Limestone, the CharmouthMudstone Formation (up to around 170 m thick)consists of fossil-rich grey mudstones with a few thinlimestones, spanning a considerable part of theSinemurian-Pliensbachian interval. This unitunderlies large areas of southern Warwickshire�s claylowland, but is very poorly exposed. In the nineteenthcentury, the Fenny Compton railway cuttings, cutthrough the clay lowland north-east of the BurtonDassett Hills, exposed fossiliferous mudstones rangingthrough parts of the Raricostatum, Jamesoni and Ibexbiozones. They were documented by Thomas Beesleyin the Proceedings of the Warwickshire Naturalists� andArchaeologists� Field Club. The cuttings are nowlargely overgrown, although small exposures ofweathered rock still provide glimpses of the site�sformer interest. In the summer of 2003, gas pipelinetrenches running for at least 4.5 kilometres north-west of Long Marston, south-east of Stratford-upon-Avon, exposed grey Charmouth mudstones. Manyammonites were found, some pyritized, andconfirmed the presence of several ammonitebiozones. Additionally, shell beds packed withbivalves (including many thick-shelled Gryphaea and

mussel-like Hippopodium), brachiopods and otherfossils were encountered (Fig. 9). Just beyond thecounty boundary, Blockley Station quarry,Gloucestershire, has long been famed as a source ofexquisitely preserved Charmouth Mudstone fossils.Recorded from Blockley, ammonites of the generaAcanthopleuroceras, Liparoceras and Tragophylloceraswere recently collected from a temporary sectionbelow the site of the former Napton on the Hillbrickworks, east of Southam. The British GeologicalSurvey have detected widespread calcareous shellyhorizons within the higher part of the CharmouthMudstone in central England, notably the so-called�70�, �85� and �100� Marker members. Such unitslocally form topographic benches on the hillsides ofsouthern and eastern Warwickshire. A Gryphaea-richlimestone bed at the top of the temporary sectionbelow Napton on the Hill might represent the �85�Marker.

The rich benthic fauna of the CharmouthMudstone in Warwickshire indicates that it wasdeposited in a relatively shallow, well-oxygenatedsea, similar in many respects to that which coveredthe region in Rugby Limestone times. Some of theshell beds seem to mark phases of increasedwinnowing, perhaps due to shallowing. The resultingcoarse-grained substrates allowed the establishment

Fig. 8. Elongate limestonenodule preserving aconcentration of ammonites(Schlotheimia). Saltford ShaleMember (Blue Lias Formation),Southam Cement Works Quarry.Warwickshire Museumcollection. Nodule is 72 cmlong.

Fig. 9. Nautiloid, dug from gaspipeline trench near LongMarston, 2003. CharmouthMudstone Formation. Scaleprovided by pencil.


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of Gryphaea-dominated communities. In this way thesediment became progressively enriched in shelldebris, facilitating further epifaunal colonization.

Overlying the Charmouth Mudstone, the DyrhamFormation (Pliensbachian; Margaritatus Biozone, upto around 65 metres thick) is marked by a higher siltand sand content, indicating shallower-waterconditions and heralding the culmination of the firstmajor Jurassic shallowing episode. The DyrhamFormation was formerly dug for brick clay at the nowlargely overgrown Napton on the Hill brickworksquarry (Fig. 10), and just beyond the countyboundary at the old Aston Magna brick pit, north ofMoreton-in-Marsh, Gloucestershire. Small exposuresof weathered micaceous siltstone and sandstone of

the Dyrham Formation are common on the steep,gorse-clad slopes of escarpments and outlying hillsthroughout the south of the county. At AvonhillQuarry, Avon Dassett, several metres of mudstone,siltstone and sandstone near the top of the formationformerly yielded many fossils, including the subzonalammonite Amaltheus subnodosus. Sawn blocks of fine-grained sandstone at this site reveal interestingsedimentary structures including burrows and shell-filled scours. The latter are suggestive of a storm-current influence in shallow water. The top of theDyrham Formation was exposed in the floor of thenow worked-out Edgehill Quarry where it included aremarkable mudstone layer packed with countlessworn belemnites (Fig. 11). This is a good example ofa �belemnite battlefield� and proves a phase of reducedsedimentation, allowing concentration of thebelemnite remains.

A widespread pebble bed, forming the floor of theEdgehill quarry, marks the base of the succeedingMarlstone Rock Formation and perhaps theculmination of the first shallowing episode. Itincludes cobbles and slabs of sandstone derived fromthe Dyrham Formation. The rusty oolitic matrixyields fossils including the typical Marlstone scallopPseudopecten equivalvis, oysters and manyrhynchonellid, terebratulid and spiriferid brachiopods(Fig. 12), adapted to life on the pebbly substrates. Thebrachiopods often form dense clusters, interpreted asremnants of brachiopod colonies on the Marlstonesea floor. Abundant belemnites are commonlycovered in the characteristic pentagonal grazingtraces of regular echinoids.

Above the pebble bed, the Marlstone at Edgehill

Fig. 10. Dyrham Formation,Napton on the Hill brickworks,date of photograph unknown.Note the line of sandstoneconcretions in the lower part ofthe face. This site is now largelyobscured, though the highestpart is preserved as a geologicalSite of Special Scientific Interest.

Fig. 11. �Belemnite battlefield�;concentration of belemniterostra at the top of the DyrhamFormation, Edgehill Quarry.Scale provided by pen.

Fig. 12. Well-preserved brachiopod (Spiriferina) from the base ofthe Marlstone Rock Formation, Edgehill Quarry. WarwickshireMuseum collection. Fossil is 37 mm wide.


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Quarry comprises several metres of unusually purechamositic oolitic ironstone, weathering to limonite(Fig. 13). Until recently it was quarried there as�Hornton Stone� for building and ornamentalpurposes, as well as a source of aggregate. It is nowgenerally thought that the iron content was derived

from laterite soils on nearby land, perhaps theLondon Platform, indicating a warm, humid LowerJurassic climate. The ironstone yields belemnites,nautiloids, brachiopods (Fig. 14), wood fragmentsand other fossils. Cross-bedding and bioturbation arewidespread. It becomes increasingly sandy andsomewhat thinner towards the Vale of Moreton Axisand also gains appreciable sand content towards thenorth-east, as at Napton on the Hill where it is visibleat the top of the disused brickworks quarry.

The overlying Toarcian Whitby MudstoneFormation (up to 60 metres thick) is very poorlyexposed. The basal beds locally contain thin bands oflimestone that yield many ammonites of the generaDactylioceras, Harpoceras and Hildoceras. This abrupt

reappearance of fine-grained ammonite-rich strataindicates early Toarcian deepening, followed byoverall shallowing in late Toarcian�Aalenian times.

Middle Jurassic

West of the Vale of Moreton Axis, near the village ofIlmington, sands of Toarcian age at the top of theWhitby Mudstone presage the development ofshallow-water oolitic and shelly �Cotswold�limestones of the Aalenian Birdlip LimestoneFormation (up to about 28 metres preserved). Thesestrata were formerly quarried at the IlmingtonDowns quarry but are now very poorly exposed. Thebasal, sandy limestones of the Leckhampton Memberpersist across the Axis, where they lie on an erodedsurface of the Whitby Mudstone. They pass furthereastwards into the Northampton Sand Formation(Fig. 3). This transitional region of southernWarwickshire attracted the attention of 19th andearly 20th century workers such as John Judd andLinsdall Richardson. Ferruginous and calcareoussandstones of the Northampton Sand are still seen inseveral disused quarries and cuttings, notably theWinderton road cutting, three kilometres north-eastof Lower Brailes. Superimposed upon the overallMiddle Jurassic shallowing trend, the transgressiveUpper Bajocian Clypeus Grit limestones cross the Valeof Moreton Axis on to the East Midlands Shelf, wherethey also come to rest on units as old as the WhitbyMudstone. Several metres of Clypeus Grit are exposedat the disused Cross Hands Quarry near LittleCompton, where it comprises buff to brown-colouredoolitic and pisolitic rubbly limestone (Fig. 15). Deep-burrowing bivalves (Pholadomya), nerineidgastropods, terebratulid brachiopods, solitary coralsand irregular echinoids dominate the rich fauna.Among the echinoids, typical large Clypeus ploti isquite common (Fig. 16). Many of the fossils found atCross Hands Quarry are from a single seam of marlylimestone, indicating a time when a stabilized seabed

Fig. 13. Marlstone RockFormation, recently exposed atEdgehill Quarry. Face is roughly5 m high.

Fig. 14. Sawn slab of HorntonStone (Marlstone RockFormation) showing cross-section through a concentrationof brachiopods. Scale providedby pen.

Fig. 15. Chipping NortonLimestone Formation overlyingthe Clypeus Grit Member, CrossHands Quarry, near LittleCompton.


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allowed a diverse invertebrate community to becomeestablished. Spoil tips of crushed rock at this siteprovide a valuable resource for fossil hunts organizedby Warwickshire Museum�s Heritage Educationservice.

The overlying Bathonian Great Oolite Group(normally no more than a few metres preserved)continues the trend of shallow-water deposition. Atthe base of the Great Oolite, the Chipping NortonLimestone rests upon upon the Clypeus Grit at CrossHands Quarry (Fig. 15), where it comprises flaggy-bedded finely oolitic limestone. An abundance ofplant debris shows that land was not far away.Farther east, closer to the London Platform, theChipping Norton Limestone passes into nearshoresands of the Horsehay Sand Formation and overstepsthe Clypeus Grit to rest on the Northampton Sand.Traces of weathered mudstone and oyster-richlimestone above the Chipping Norton Limestone atCross Hands Quarry represent the lower part of theoverlying Sharp�s Hill Formation. Warwickshire�syoungest Jurassic beds, mapped as the late BathonianForest Marble Formation, were detected by the BritishGeological Survey at Broomhill Farm, south of UpperTysoe.

Conclusions

Warwickshire�s Lower and Middle Jurassic stratademonstrate many lines of evidence for the existenceof marine environments in central England betweenabout 205 and 165 million years before present. Theoverall stratigraphy and palaeontology wasestablished during the 19th and early 20th centurieswhen the rocks were widely exposed in quarries andcuttings. The remaining sites and temporaryexposures continue to reveal new features ofgeological importance, notably fossils, addingconsiderably to our knowledge of central Englandduring Jurassic times. Prospects for furtherdiscoveries are good, given the current level ofinterest in Warwickshire�s geology bygeoconservation specialists, researchers and amateurgroups.

Acknowledgements

Keith Ambrose (British Geological Survey) and MikeSumbler (formerly British Geological Survey) kindlycommented on an early draft of this article.

Suggestions for further reading

Ambrose, K. 2001. The lithostratigraphy of the BlueLias Formation (Late Rhaetian-Early Sinemurian)in the southern part of the English Midlands.Proceedings of the Geologists� Association, v.112,

Fig. 16. Clypeus ploti � anirregular echinoid from theClypeus Grit of Cross HandsQuarry. Warwickshire Museumcollection. Specimen is 97 mmin diameter.

pp.97�110.Beesley, T. 1877. The Lias of Fenny Compton,

Warwickshire. Proceedings of the WarwickshireNaturalists� and Archaeologists� Field Club, 1877,pp.1�21.

Cox, B.M. & Sumbler, M.G. 2002. British MiddleJurassic Stratigraphy. Geological Conservation ReviewSeries. Joint Nature Conservation Committee,Peterborough.

Edmonds, E.A., Poole, E.G. & Wilson, V. 1965.Geology of the country around Banbury and Edge Hill.Memoir of the Geological Survey of Great Britain.HMSO, London.

Hallam, A. 1968. The Lias. In: Sylvester-Bradley, P.C.& Ford, T.D. (eds). The Geology of the East Midlands.Leicester University Press, pp.188�210.

Hallam, A. 1975. Jurassic Environments. CambridgeUniversity Press.

Horton, A., Poole, E.G., Williams, B.J., Illing, V.G. &Hobson, G.D. 1987. Geology of the country aroundChipping Norton. Memoir of the British GeologicalSurvey. HMSO, London.

Old, R.A., Hamblin, R.J.O., Ambrose, K. &Warrington, G., 1991. Geology of the countryaround Redditch. Memoir of the British GeologicalSurvey, HMSO, London.

Old, R.A., Sumbler, M.G., & Ambrose, K. 1987.Geology of the country around Warwick. Memoir ofthe British Geological Survey, HMSO, London.

Radley, J.D. 2003. Warwickshire�s Jurassic geology:past, present and future. Mercian Geologist, v.15,pp.209�218.

Simms M., Chidlaw, N., Morton, N. & Page, K.N.2004. British Lower Jurassic Stratigraphy. GeologicalConservation Review Series. Joint NatureConservation Committee, Peterborough.

Williams, B.J. & Whittaker, A. 1974. Geology of thecountry around Stratford-upon-Avon and Evesham.Memoir of the Geological Survey of Great Britain.HMSO, London.

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Highlights of Jurassic Warwickshire