OXFORDSHIRE ROCKS | MARLSTONE ROCK FORMATION
Pliensbachian Age - Toarcian Age.
marl: a calcareous mudstone.
Image: Marlstone in hand specimen - £1 coin for scale (Ⓒ Dexter Brown)
* Ma is an abbreviation for million years
The dark brown to reddish brown Marlstone Rock Formation crops out in North Oxfordshire around Banbury. It's an iron-rich (ferruginous), fossil-rich ooidal (composed dominantly of ooids - ooids are small spheroidal, sedimentary grains, usually composed of calcium carbonate), limestone. It can be interbedded with cemented (calcareous) sandstone and ferruginous mudstones. In some locations it may be so iron-rich that can be described as ooidal ironstone. It contains abundant marine fossils. The Marlstone is usually well stratified and well sorted, and usually cross-bedded.
The iron content (as ooids, altered shell material or in the groundmass) is berthierine (dark green iron-rich mineral formed in low-oxygen marine conditions), altering to siderite (iron carbonate).
There are two distinct facies (a sedimentary facies is a distinctive rock unit that forms under certain conditions of sedimentation, reflecting a particular process or environment):
berthierinic/sideritic oolith-shell fragmented limestones
thinner more sideritic, sandy limestones.
The former facies provides the economically valuable iron ore.
The base of the formation is determined at the downward change to mudstones and siltstones of the Dyrham Formation. The transition is usually erosive and conglomeratic band up to 0.3 metres thick, which consists of phosphatic or ferruginous tabular pebbles of limestone or mudstone. In some places the formation may rest on a sandstone bed at the top of the Dyrham Formation, which it may be impractical to map separately from the Marlstone Rock.
The boundary has been established at the point where there is a transition to the mudstone/nodular limestones of the Whitby Mudstone Formation.
The formation is heavily cambered and for this reason thickness estimates can be unreliable. Isopachyte maps put the maximum thickness between 7.6 m - 10 m (at Bloxham).
In the Middle Lias there was a general lowering of sea level, resulting in a progressive regression. The sedimentation of the Marlstone Rock Formation therefore most likely occurred in a series of relatively shallow, interconnected basins between the London Platform and the Severn Basin. This would account for the variations in thickness.
The faunal species are predominately sessile (organisms that are fixed in one place; immobile eg barnacles) sea floor dwellers. Burrowing species are absent, quite probably because the ooidal sands would have shifted regularly in proximal zones (close to shore) and more turbulent storm phases. The faunal assemblage is dominated by typically of nested of rhynconellid, Tetrarhynchia tetrahedra and Lobothyris punctata brachiopods. Several species of the ammonite Pleuroceras, and numerous belemnites and bivalves, rare corals, crinoids and plant material are also present. The zone index fossil Pleuroceras spinatum is rare, but has been found at Redlands Quarry, Hook Norton and at Fawler. Belemnites occur abundantly in the conglomeratic bed at the base of the formation, but are rare later in this deposit.
The Marlstone Rock Formation outcrops widely across northern Oxfordshire and east of the Cherwell Valley.
The formation, which is more resistant to erosion than the over-lying Upper Lias strata, characteristically forms a shelf or bench along the lip of the scarp face and along valley sides. I t forms an extensive plateaux between Deddington, Adderbury, Wroxton and Hook Norton. The Sor Brook and its tributaries, drains the marlstone plateaux in a south-easterly direction towards the Cherwell, cutting deep, down through the Middle Lias units to the Charmouth Mudstone Formation.
The formation is present from within the Amaltheus margaritatus to the end of the Dactylioceras tenuicostatum ammonite zones.
Known as locally as Hornton Stone or Banbury Ironstone.
Building stone (ashlar and rubble). Much of the stone that has been quarried, was used for purposes other than architectural. Many of the villages of the Banbury, Wroxton and Deddington district owe their special character to its rich gingerbread hues. In Banbury Samuel Pepys Cockerill's church of St Mary (1797) is resplendent Hornton ashlar. In Oxford, Deane & Woodward used the stone for string courses and ornamental work round the windows of the University Museum (1859) and Christ Church Meadow buildings (1862-65). Butterfield also used Hornton Stone together with Bristol sandstone in a chequer-effect in the chapel at Balliol College.
From the late 19th through the first half of the 20th centuries it was predominantly a source of iron ore. The exploitation of the Marlstone Rock Formation for iron ores began in the 1850's, with pits opened at Fawler in the west of the area and sites along the Cherwell Valley eg. Steeple Aston and Adderbury to the east. These sites were identified as viable because of the proximity of local rail lines. Later the Oxfordshire Ironstone Railway and the King's Sutton to Cheltenham GWR line gave rise to the industrial scale quarries at Hook Norton, Bloxham, Wroxton and Alkerton .
Berthierine is a dark olive green to yellowish green mineral. It is an iron-rich, aluminous, 1:1-type layer silicate belonging to the serpentine group.
Siderite is a dark to light brown (sometimes yellow/grey/white). It is iron-rich (Iron carbonate) and belongs to the calcite group of minerals.
Image 1: Exposure at Bloxham Quarry ca 1960 (© BGS)
Image 2: Photomicrograph showing rolled berthierine flakes, ooliths and false ooliths set in a sparry calcite matrix. Some of the calcite material is fossil debris. PPL x125, av grain size 0.14 mm from BGS borehole 21 at 5.3 m depth. (© BGS)
Is the branch of stratigraphy that uses fossils to correlate and establish relative ages of rock units.
A scientific adjective meaning mostly or partly composed of calcium carbonate: lime-rich or chalky.
A sedimentary rock that is composed of a substantial fraction of rounded to sub-angular fragments of older, eroded rocks.
The Marlstone Rock Formation as a source of building stone.
Known locally to masons, builders and the general public as Hornton Stone or Banbury Ironstone, It weathers to a distinctive, golden-orange to brown colour but can appear bluish-green when freshly cut or unweathered.
This building stone is a 'rubble' stone or 'rag' stone' (fragments of stone that can be laid as uncoursed work).
The stone is susceptible to weathering and spalling, and in places it has been replaced with alternative materials.
The sienna/caramel coloured Hornton Stone characterises the vernacular architecture of many of the local settlements, for example, Wroxton, Banbury, South Newington, Sibford Gower and Deddington owe their charm to its warm hues.
Image 3: St Mary's Church, Banbury (Hornton stone ashlar blocks)
Image 4: cottages in Hornton.
Exploiting the ironstone
It is now hard to envisage that the pleasant rolling Cotswold countryside of north Oxfordshire was once host to opencast quarrying of the Marlstone Rock Formation for iron and steel production on an industrial scale.
The excavation of ironstone started between Banbury & Hook Norton in the west and Adderbury and Deddington in the south of the area in 1819. Economic pressures finally forced closure of the last quarries in 1967.
The ironstone was moved by rail from the pits to the distant steelworks firstly over the private mineral railway of the Oxfordshire Ironstone Company (OIC) to mainline railway sidings a tad north of Banbury. The planned GWR Banbury-Cheltenham railway plans were re-routed, presenting significantly demanding challenges to the engineers, in order for the line to run through Hook Norton to facilitate ironstone shipping to South Wales and the English Midlands.
Over the life time of the quarries some 33 million tons of iron ore was removed.
Image 5: The Oxfordshire Ironstone Railway (OIR) near Balscott. (© Banbury Museum).
Image 6: Fourth Kiln at Brymbo Iron Works, Hook Norton taken in 1922. (Henry Simms, Packer Collection © Oxfordshire County Council Photographic Archive [D243329a]).
Ironstone quarrying at Hook Norton
The earliest that serious quarrying started at Hook Norton seems to be 1889. Hook Norton Ironstone Partnership was the first to quarry ironstone at Hook Norton on a large scale. Twelve years later it became part of the Brymbo Ironworks and Quarries of Wrexham, which established a new quarry at Hook Norton in 1899.
The main area operated (known as Redlands Quarry) lay north of the main road, stretching from the present Hollybush Road up to Redlands Farm. The overburden of clays and fissile limestones of the Whitby Mudstone Formation was particularly shallow here. It was estimated that there were 290,000 tons of ore available, two-thirds of it at Redlands, plenty enough to last thirty-six years. This quantity exceeded Brymbo’s requirements, providing a surplus that was sold to foundries in the South Wales and the Midlands.
Image 7: Redlands Quarry, Hook Norton [360 347] Marlstone Rock Formation overlain by the Whitby Mudstone Formation. (© British Geological Survey)
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