Portland cement is the most common type of Cement in general use around the World as a basic ingredient of Concrete, Mortar, Stucco, and non-specialty Grout. It was developed from other types of hydraulic Lime in England in the early 19thC by Joseph Aspdin, and usually originates from Limestone. It is a fine Powder, produced by heating Limestone & Clay Minerals in a Kiln to form Clinker, grinding the Clinker, and adding 2 to 3% of Gypsum. Several types of Portland Cement are available. The most common, called ordinary Portland Cement (OPC), is Grey, but White Portland Cement is also available. Its name is derived from its resemblance to Portland Stone which was Quarried on the Isle of Portland in Dorset. It was named by Joseph Aspdin who obtained a Patent for it in 1824. However, his son William Aspdin is regarded as the Inventor of “Modern” Portland Cement due to his developments in the 1840s England remained the main source of the World’s Cement during the 1850s & 1860s. This Export Market allowed the English Plants to expand Production & mature their Technology, and the Home Market for the Product began to take off from 1860 onward, rapidly overtaking that of the earlier Products. Exports all around the Globe remained a major part of the Industry’s Business until the 1890s, when it suddenly became apparent that Foreigners & Colonials could make Portland Cement as well.
The original “invention” or “discovery” of Portland Cement sets the standard for all subsequent Cement Industry history, in that it cocooned in a web of disinformation, misunderstanding & lies. In this work, the term Portland Cement is applied only in the modern sense. “Portland Cement as we know it” can be simply defined as a calcareous Cement containing (intentionally!) significant amounts of alite (“tricalcium silicate”). Portland Cement is one of many materials that can be made by making a finely-ground mixture of Limestone & Clay, and burning the mixture at over 1000°C. The properties of the product obtained differ widely depending on the composition of the mixture and the temperature of burning.
Chinnor Cement & Lime Co 1937
The most important Industrial undertaking in the Parish has been A large Cement Works was established as the Chinnor Cement & Lime Co Ltd. It was Founded in 1908, became a Public Company in 1936 & in 1949 a Parent Company, Chinnor Industries Ltd, with 3 Subsidiary Companies. The Founder Mr William Elijah Benton a Mining Engineer from Acton, started a small Business to manufacture Lime on the Southern edge of Chinnor. This eventually consisted of 5 Beehive Lime Kilns producing Lump Lime for use in both Construction & Agriculture. In 1919 he added a small Cement Plant which used Flare Kilns (that had to be loaded & unloaded by hand). By 1928 the Business had grown and the 1st Rotary Kiln was commissioned and following Progress & Development, the Chinnor Cement & Lime Co. Ltd was formed. In 1936 further investment followed, with Rotary Kiln No.2 being installed in 1938. In 1957 it Employed about 160 men, but the Works were being extended with the object of doubling its Capacity and bringing it to a still higher degree of Efficiency. Its Market was a comparatively Localised one, the majority of its Customers being in the 7 Counties which were closest to the Works. A further Rotary Kiln No.3 was added in 1958. The Company had also grown, becoming Chinnor Industries Ltd; by 1962, the original Rotary Kiln had been replaced by a new No.1 Inclined Rotary Kiln and the Site extended to almost 200 acres.
Chinnor Cement Works with Lime Kilns in the Background
Chinnor Cement Quarry:
Chalk has been extracted for Cement making from this Quarry since 1908. At the height of Production, the Quarry manufactured 5,600 Tonnes per week. It closed in 1999, its life extended by a sudden need for more Cement for the Channel Tunnel. In fact, the Boring Machine was Tested here before tackling the Tunnel. A 210M long Tunnel, 5M in dia, was bored into the Grey Chalk close to the Site in 1974 in order to evaluate methods of investigating Ground conditions for the future Channel Tunnel. Cement manufacture requires a vast amount of Lower Chalk which, unlike the Middle & Upper Chalk, is not hard, but is a soft Chalky Clay. Extraction leaves large Pits that fill with Rainwater which looks Blue due to fine particles in suspension.
(Caution! DO NOT enter the Quarries: they are Dangerous and Privately Owned).
The Chinnor Chalk Pit Site comprises Chinnor Quarry No.3, the most South-Eastern and most extensive, of the Complex of Quarries belonging (until 1999) to Rugby Group Plc. The Site exposes a continuous, Fossiliferous Section at the Southern end of the Chiltern Hills through the greater part of the Grey Chalk Subgroup, as well as the Lower part of the Holywell Nodular Chalk Formation at the Base of the White Chalk Subgroup. Together with other Quarry Sections in the Chiltern Hills.
Chinnor Chalk Pit (Quarry No.3) is a narrow, Deep Quarry, 950M long, orientated NE-SW, situated between the Icknield Way and the Lower Slope of the Chiltern Hills Scarp. Access is through a Tunnel beneath the Icknield Way. The Quarry is c.100M wide at the South-west-end, which is partially Flooded & c.200M wide at the North-east-end, where there are 7 Faces, each 5-6M high, separated by ‘Benches’. The orientation of the Quarry is such that the Faces expose approximately Dip & Strike Sections. The Quarry exposes c.35M of the Grey Chalk Subgroup, which is known from Borehole evidence to be c.60M thick here, and the lowest c.14M of the Holywell Nodular Chalk Formation at the Base of the White Chalk Subgroup.
The Structure is simple, the Regional Dip, and the Overall Dip of the succession exposed in the Quarry, being only c.0.5º-1º to the South-East; however, the higher Beds exhibit opposite Dips (up to 8º-11º) to the North-West. Only minimal Faulting can be observed in the Strike-Sections, but the Dip-Section at the North-East-end is extensively affected by closely spaced normal Faults, both oblique & sub-parallel to the Face. The Plenus Marls Member is affected by normal Faults orientated approximately North–South, with displacements of 0.5M to over 2M to the East. The only Fault that can be traced throughout the North-east Faces has a throw of 2M in the Plenus Marls, reducing to 0.4M at the Level of the Totternhoe Stone.
The Grey Chalk Subgroup Section provided a Log of the lowest Beds in the present Quarry. A 210M long Tunnel, 5M in dia, was Bored into the Grey Chalk close to the Site in 1974 in order to evaluate methods of investigating Ground conditions for the planned Channel Tunnel.
The succession extends from a Level in the West Melbury Marly Chalk Formation of the Grey Chalk Subgroup, 13.5M beneath the Totternhoe Stone, up to the beginning of the Shell-detrital Chalks in the Holywell Nodular Chalk Formation, near the Base of the White Chalk Subgroup. This figure is a composite of data from the British Geological Survey Log, a Log of the succession beneath the Totternhoe Stone and an unpublished Log of the Holywell Nodular Chalk. The Moghadam & Paul Log shows an additional 2.5M beneath the Base of the British Geological Survey Log, different details of the highest Beds beneath the Totternhoe Stone, and a measurement of c.21M, from the Base of the Totternhoe Stone to the Base of the Plenus Marls Member, as against 24M.
Grey Chalk Subgroup
The Lower Chalk of Chinnor was recently divided by the British Geological Survey into the 4 Traditional Lithostratigraphical Units applicable in the Chiltern Hills. These were the Chalk Marl (c.11M exposed), Totternhoe Stone (c.1M), Grey Chalk (c.23M) & Plenus Marls (c.1M). However, the Chalk Marl in the Chiltern Hills represents the part of the West Melbury Marly Chalk Formation of the Southern Province that is preserved beneath the sub-Totternhoe Stone erosion surface. The Totternhoe Stone itself equates with the basal part of the Zig Zag Chalk Formation, the overlying ‘Grey Chalk‘ constituting the remainder of the Formation. The preserved West Melbury Marly Chalk exhibits the typical succession of well-differentiated Marl–Limestone precession couplets that is seen in the standard Southern Province Sections. There was a marked upward change, at c.9M below the Totternhoe Stone, from White Chalks to Grey Chalks. The thin (c.1M) brown-coloured, calcarenitic, sandy Totternhoe Stone, developed here in Shelf Facies, rather than Channel Facies, stands proud in the Faces and can be followed throughout the Length of the Quarry. The Base of the Stone, particularly in the South-Western part of the Quarry, rests with sharp, undulating & slight erosive contact on the underlying Beds. The coarse-grained Sediment is piped down in a Thalassinoides Burrow System for up to 0.5M below the contact. Phosphatised & Glauconitised pebbles, including internal moulds of Ammonites, are concentrated at the Base, and small phosphatic intraclasts & Fish Debris are found throughout. The main mass of the Stone contains the trace Fossil Teichichnus. The top of the Stone is gradational and is marked by the Upper Limit of phosphatic intraclasts. The overlying Beds of the Zig-Zag Chalk Formation cannot be readily examined in the steep Faces. The sediments of the lowest 2M above the Totternhoe Stone are silty and distinctly pale brown in colour, with scattered Pyrite nodules. The greater part of the overlying succession comprises poorly differentiated & relatively inconspicuous alternations of thin, slightly darker coloured, Mioturbated Marls & thicker units of Off-white Chalk. A massive Bed, 2M thick, of slightly silty Chalk with scattered Pyrite Nodules, with a 0.50M thick Marl at its Base, c.7M above the Totternhoe Stone, and overlain by a closely spaced Marl-pair is inferred to equate with Jukes-Browne Bed of the Basinal successions. In the highest 4M beneath the Plenus Marls, the differentiation into Marls & Chalks becomes more noticeable. The wavy Basal contact between Beds D2 & D3, c.4M beneath the Plenus Marls, may represent a scoured surface, or even a regional erosion surface.
White Chalk Subgroup
The relatively dark-coloured Plenus Marls Member forms a conspicuous marker Horizon in the higher part of the Quarry and emphasizes the effects of the Faulting. Compared to its development in Sections in the Central & Northern part of the Chiltern Hills, the Member is extremely thin here (up to 1M, depending on the degree of compaction), and the standard 8 Beds, particularly Beds 4–8 inclusive, cannot always be readily identified. Burrows extend down from the undulating sub-Plenus erosion surface for up to 0.5M, but are concentrated in the top 0.1M. There is an excellent exposure of the Basal Beds of the Holywell Nodular Chalk Formation above the Plenus Marls Section, and inaccessible exposures of even higher Beds in the same formation. The highest Beds appear to belong to the interval, 2/3rds of the way up this Formation, that is composed of the most Shell-detrital-rich sediments.
Chinnor Quarry is a Geological Site of Special Scientific Interest which exposes fine Sections of the Lower Chalk and the Basal part of the Middle Chalk. After a brief outline of the nomenclature & stratigraphy of the Chalk succession of the area, the Quarry Section is described in more detail, with particular reference to the vertical distribution of Macrofossils and their significance in Zonation of the sequence.
The Lowest Beds exposed, which are best seen at the South-west-end of the Quarry, lie in the lower part of the Lower Cenomanian Mantelliceras dixoni Zone. Even Lower Beds, formerly exposed in deep Trenches at the Base of Quarry No.2, yielded an Ammonite assemblage dominated by Ammonoid Cephalapods with subordinate Mariella & Schloenbachia. This important material, which is Housed in the University Museum, Oxford, must have its provenance in the Mantelliceras saxbii Erosion Event Bed in the higher part of the underlying Mantelliceras saxbii Subzone of the Mantelliceras mantelli Zone. This Bed equated with a Bed, c.15M above the Base of the Grey Chalk Subgroup, that had been Mapped in the surrounding area. In the lowest part of the Section, a cemented Limestone containing common 3-dimensional Inoceramus ex gr. virgatus Schlüter with the bi-valves associated represents the Dixoni Limestone Marker Horizon of Sections in the Northern Chiltern Hills. This Bed correlates with a pair of closely spaced spongiferous Limestones in the Folkestone to Kingsdown section, which likewise, marks the top of an interval in which the Inoceramid assemblage comprising with subordinate Inoceramus crippsi Mantell, is increasingly dominated upwards by Inoceramus ex gr. virgatus. The Dixoni Limestone at Chinnor is very fossiliferous, having yielded, in addition to the Inoceramid Bivalves (Clams), Serpulids, the long-ranging bivalves Plagiostoma globosum & Plicatula inflata and the stratigraphically relatively restricted small brachiopod Monticlarella? rectifrons (Pictet & Campiche). The only Ammonite recorded so far is a large unidentified Acompsoceras sp. A 0.3M Bed of spongiferous Limestone, some 3M beneath the Base of the Totternhoe Stone, yielded 3 specimens of the heteromorph Ammonite Turrilites scheuchzerianus Bosc, enabling correlation with the scheuchzerianus event Bed at Southerham Grey Pit. This Marker Horizon is not readily identifiable in the published Section, but may, equate with the lower part of Bed H21, rather than with the slightly silty Bed H23. As a result of sub-Totternhoe Stone erosion the basal Middle Cenomanian Ammonite Zone of Cunningtoniceras inerme appears to be completely missing (see below). The Totternhoe Stone is very fossiliferous (collecting is best from fallen Blocks) and is well known to Local Collectors for its Vertebrate remains, including Bones of Flying Reptiles (Pterosaurs) & large Vertebrae of Lamnid Sharks. A Turtle Humerus was collected from this Bed in the Pitstone Quarry. The Totternhoe Stone at Chinnor also yields abundant Teeth of Sharks, including Mackrel Shark and species of Cow Shark & Lamin Form Shark, with smaller numbers of Shell Crushing Shark. The Totternhoe Stone is inferred to represent, in condensed form, the Turrilites costatus Subzone of the Middle Cenomanian Acanthoceras rhotomagense Zone. The Base of the Stone here contains well preserved 3-dimensional phosphatised internal Moulds (steinkerns) of Ammonites, including Acanthoceras rhotomagense (Brongniart), Cunningtoniceras sp., Schloenbachia coupei (Brongniart), Sciponoceras baculoides (Mantell) & Turrilites costatus Lamarck. The Totternhoe Stone also yields poorly preserved unphosphatised composite Moulds of large Ammonites such as Acanthoceras & Parapuzosia (Austiniceras), which represent indigenous faunal elements. As in other Localities in the Chiltern Hills, the small, coarsely ribbed rhynchonellid brachiopod Orbirhynchia mantelliana occurs in profusion throughout the Totternhoe Stone, and a rich indigenous fauna of Bivalves (notably Plagiostoma globosum, Plicatula inflata, various Oysters and the thin-shelled pectinacean Entolium orbiculare) & terebratulid brachiopods is found concentrated at the Base. The geographically widely distributed, but stratigraphically restricted Belemnite (Squid & Cuttlefish), Praeactinocamax (formerly Actinocamax) primus (Arkhangelsky) also occurs. The Beds immediately overlying the Totternhoe Stone contain the terebratulid brachiopod Concinnithyris subundata, which elsewhere characterises the succeeding Turrilites acutus Subzone. The higher Beds are relatively poorly fossiliferous. The Marls immediately overlying the inferred equivalent of Jukes-Browne Bed 7 contain small Oysters (Amphidonte sp.), some with atttachment areas moulding the inoceramid Bivalve Inoceramus pictus. However, the thin Marl with sparse small pycnodonteine Oysters that are found elsewhere in the Chiltern Hills (e.g. Totternhoe Quarry) at the Base of this Bed has not so far been identified. This Oyster Horizon may be represented by part of the interval of Marly Chalk with Marls comprising Beds F3 to F5, or it may be situated in the immediately overlying obscured part of the Section at the foot of the next face. Amphidonte sp. together with common specimens of the thin-tested echinoid Sternotaxis gregoryi, are also found some Metres higher, in Bed D1. Although well exposed, the very condensed Plenus Marls Member has yielded only a limited fauna. As usual, small to medium-sized Oysters (Pycnodonte) and the large rhynchonellid brachiopod Orbirhynchia multicostata Pettitt are common in the Basal Marly Jefferies’ Bed 1, and the eponymous Belemnite, Praeactinocamax (formerly Actinocamax) plenus can be collected from the silty Jefferies’ Bed 4. The extremely indurated (topmost Cenomanian) Limestones overlying the Plenus Marls Member are readily accessible and, as elsewhere, contain the straight heteromorph ammonite Sciponoceras bohemicum anterius Wright & Kennedy and spines of the regular echinoid Hirudocidaris hirudo (Sorignet). Rhynchonellid brachiopods are characteristically absent from this interval everywhere. The reappearance of Orbirhynchia in a silty Bed, 1.8M above the Plenus Marls, is an important Lower Turonian Bio-event that can be used for correlation with Sections in the Southern Province (see below). The highest Beds in the Quarry are Shell-detrital Chalks rich in fragments and complete Valves of the Inoceramid Bivalve Mytiloides.
The preserved thickness of the West Melbury Marly Chalk Formation below the Totternhoe Stone is inferred from Boreholes to be between 36 & 40M. This thickness compares with a similarly inferred thickness of c.50M at Pitstone Quarry, 23 Km to the North-east, and, in the Northern part of the Chiltern Hills, the 54M proved below the Totternhoe Stone by the British Geological Survey Sundon Borehole in the Base of Sundon Quarry.
Sundon Quarry Operation. The Upper Dragline removes the top part of the Gray Chalk and drops it onto the Bench below. The Face Shovel (Ruston Proctor 1909) digs the Lower part of the Face, and loads the Train with this and the top material thrown down. The Bench Level is adjusted so that the Face Shovel is placed at the optimum Level to prise out the Hard Band of Totternhoe Stone. The Lower Dragline adds Lower Carbonate Marl to the Train. The Chemistry of the mixture is controlled by varying the number of Grabs of Lower Marl per Train-load. As the thickness of the Gray Chalk increases, so the depth of the Lower Pit automatically increases to compensate. All Surface Water drains away into a Sump in the Lower Pit. This system of advance automatically adjusts itself to the Geology of the deposit without Technical Supervision. The Quarry was operated by 3 Excavator Drivers and 2 Locomotive Drivers.
To place this expansion in context, the thickness of the entire Formation in the Folkestone to Kingsdown Cliff section is only of the order of 34M. The greatest expansion in the West Melbury Marly Chalk Formation in the Chiltern Hills is actually found in the succession below the Dixoni Limestone and, particularly, in the succession below the Doolittle Limestone, including the Basal Beds with Aucellina (Scallop).
Correlation of the Cenomanian Grey Chalk Subgroup from Chinnor Chalk Pit to other Sites in the Transitional Province and a comparison with the Folkestone Standard Section. (G=Glauconitic Marl; JB7=Jukes-Browne Bed 7; M3=Marker Horizon 3)
The occurrence at Chinnor in Quarry No.2, at a Lower Stratigraphical Level than the Lowest Beds exposed in Quarry 3, of the event Bed with Mantelliceras saxbii is noteworthy. This Bed must fall in the interval between the Dixoni Limestone & the Doolittle Limestone. Chinnor Chalk Pit is one of only 3 Localities in England where this Bed, situated in the higher part of the Mantelliceras saxbii Subzone of the Mantelliceras mantelli Zone has been recognised, the other Localities being Southerham Grey Pit, Lewes & Compton Bay, Isle of Wight. Elsewhere, this Bed has either been removed by the Erosive Event that preceded the deposition of the overlying Mantelliceras dixoni Zone sediments, or it is not sufficiently cemented to preserve Ammonites. The distinctive Event Bundle, within the Mantelliceras dixoni Zone, comprising the strongly cemented, Sponge-rich Dixoni Limestone, an overlying Bed of Marly Chalk with small brachiopods (locally including Orbirhynchia mantelliana), and a conspicuous Dark Marl, is readily recognisable near the Base of the Section. The Brachiopod Bed equates with the Lowest of the 3 Horizons with Orbirhynchia mantelliana in the Grey Chalk Subgroup of the Southern Province, and with the Lower of the 2 Horizons in the condensed Ferriby Chalk Formation of the Northern Province. However, in contrast to the other Chiltern Hills Sections, Orbirhynchia mantelliana has not so far been found here in this Bed. The Dark Marl, seen particularly well in the Folkestone Section, is an excellent Marker Horizon throughout the Chiltern Hills.
The composite Event Bundle enables Chinnor Chalk Pit to be directly correlated with standard Southern Province successions (e.g. Folkestone to Kingsdown) and with sections in the Northern Chiltern Hills, for example Sundon Quarry & Barton-le-Clay Quarry. Two developments of the Totternhoe Stone can be distinguished in the Region around Chinnor Chalk Pit. The 1st is a thin (less than 1M thick), highly fossiliferous Bed, with numerous Orbirhynchia mantelliana and small phosphatic intraclasts scattered throughout (the ‘Shelf Facies’). The 2nd is a much thicker, relatively poorly fossiliferous, ‘Channel Facies’, characterised by the trace fossil Teichichnus. It is the ‘Shelf Facies’ that is found at Chinnor Chalk Pit. The Channel Facies is up to 4.7M thick at the type locality in the Totternhoe Stone Pit, within the Totternhoe Quarry and it is separated from the Shelf Facies elsewhere in the Quarry by a distance of 200M or less. North of Hitchin, at Arlesey (Green Lagoon Pit), the Channel Facies comprises a complex succession, c.6M thick, of calcarenites overlain by calcisiltites, whereas the Shelf Facies in the adjacent ‘Blue Lagoon’ Pit, only 300M away, consists of only c.1M of calcarenites. The Channel Facies always involves a greater extent of pre-Totternhoe Stone erosion than the Shelf Facies: in the Totternhoe Stone Pit, erosion has cut down into the higher part of the Aucellina Beds. The Totternhoe Stone Section at Chinnor Chalk Pit can be linked in a network to several other Sections in the Chiltern Hills. These include the type locality, Totternhoe Lime Quarry, where both the Shelf & Channel Facies are developed, Houghton Regis Quarry, Barton-le-Clay Quarry, the 2 adjacent Sections at Arlesey, ‘Green Lagoon’ & ‘Blue Lagoon’ which exhibit the Channel & Shelf Facies respectively & Barrington Chalk Pit. At Chinnor Chalk Pit, the Basal contact of the Totternhoe Stone is situated c.5M above the Dixoni Limestone, and only c.3M above the event Bed with Turrilites scheuchzerianus, which is found elsewhere (e.g. Southerham Grey Pit) in the higher part of the Mantelliceras dixoni Zone. This means that the Basal Beds of the Middle Cenomanian (Cunningtoniceras inerme Zone), and the highest part of the Lower Cenomanian dixoni Zone (an interval of c.6-7M in the Southern Province Sections), are missing at Chinnor Chalk Pit . Farther to the North, pre-Totternhoe Stone erosion has cut down much deeper, for example in the Totternhoe Quarry, where the Base of the Shelf Facies of the Totternhoe Stone rests on the Dixoni Limestone itself, while in the nearby Stone Pit the channel facies rests on a level near the base of the Grey Chalk Subgroup. The sub-Totternhoe stratigraphy at Houghton Regis is somewhat difficult to interpret, but at Barton-le-Clay Quarry, the basal contact is only 3.5M above the Dixoni Limestone. Moghadam & Paul show a *13C curve that does not exhibit the double-peaked positive excursion that is found above and below the Cast Bed at Folkestone & correlative Localities. They use this absence to suggest that the Totternhoe Stone has eroded down from above the Cast Bed. They also identified a sudden increase in the proportion of planktonic foraminifera in the assemblage at the higher of 2 conspicuous Marl Seams c.3M above the Base of the Totternhoe Stone and correlated this with the so-called ‘P/B break‘ in the Southern Province. On this basis, the Boundary between the Turrilites costatus & T. acutus subzones of the Acanthoceras rhotomagense Zone must lie at, or slightly higher than, this Level. Bed E1, which has a slightly gritty texture, corresponds to the Chiltern Hills equivalent of Jukes-Browne Bed 7 of the Southern Province or, alternatively, to a very poorly lithified & ill-defined development of the Nettleton Stone of the Northern Province. It is underlain by 3 Marls from which Moghadam & Paul recorded the Oyster Pycnodonte, confirming the position of the Pycnodonte Event of Northern European Event Stratigraphy, and it is overlain by a pair of Marls containing the Oyster Amphidonte. This latter Oyster occurrence appears to correlate with the Lower of 2 Amphidonte Events recognised in Westphalia, Northern Germany at this Level. The higher Amphidonte occurrence, associated with the Inoceramid Bivalve Inoceramus pictus and the thin-tested echinoid Sternotaxis gregoryi, in Bed D1, probably correlates with the higher of the 2 German Amphidonte Events. The glauconitised/phosphatised Pebble Bed in a coarse-grained Chalk Matrix (the ‘Buckinghamshire Rag‘ of the earlier literature), that is locally developed at this Horizon to the North-east in Pitstone Quarry No.2 and, associated with abundant thick-shelled terebratulid brachiopods (Ornatothyris sulcifera), in the Totternhoe Lime Quarry and the former Grove Mill Quarry, Hitchin, is not found at Chinnor. In marked contrast to the relative expansion in the Chiltern Hills of the preserved West Melbury Marly Chalk Formation, the Zig Zag Chalk Formation, particularly the interval from the Base of the Totternhoe Stone to the Base of the equivalent of Jukes-Browne Bed 7, is conspicuously thinner than in Southern Province successions. This is largely due to the strong condensation represented by the Tottternhoe Stone itself. The Interval from the top of the Jukes-Browne Bed 7 equivalent to the sub-Plenus erosion surface (i.e. the White Bed of the North Downs), on the other hand, retains a more or less constant thickness of the order of 15–16M from the North Downs to Chinnor Chalk Pit. At the former Butler’s Cross Quarry, 11 Km to the North-east of Chinnor, the interval from the Base of the Plenus Marls Member to the inferred correlative of the Buckinghamshire Rag, is only some 5M in extent. This could be interpreted as the result of pre- Plenus Marls erosion.
Bacombe Hill Nature Reserve. (There’s no sign for this Nature Reserve as you enter via the Gate). Go straight ahead, through the Gate and follow an obvious Path until you meet a T-junction with the Ridgeway Path. The depressions and mounds in the lower part of this Reserve are the workings of the old Chalk Pits.
However, at the Pitstone Quarry No.2 RIGS Site, where the Buckinghamshire Rag is locally developed, the sub-Plenus erosion surface is situated less than 1M above a Hardground. The 13M Section in the Holywell Nodular Chalk Formation above the Plenus Marls Member is potentially of importance in establishing the correlation with the relatively condensed succession in the North Downs and the more expanded succession at Beachy Head, Eastbourne, using the Marker Horizons documented by Gale. Compared with Dover, the Basal Beds (Gale’s Ballard Cliff Member) are only 0.7M thick as against 1M. The lowest 2 couplets belong to the highest part of the Metoicoceras geslinianum Zone, and the remainder of the Unit can be inferred to belong to the overlying terminal Cenomanian Neocardioceras juddii Zone. The top of this Unit marks the approximate position of the Cenomanian–Turonian Boundary and the inferred base of the Watinoceras devonense Zone. The Calcarenitic Bed that is used in the Chiltern Hills & the Hitchin Area as a correlative of the Base of the Holywell Marl 2 – Holywell Marl 3 interval can be identified 1.8M above the Plenus Marls Member. This Bed marks the 1st occurrence of Orbirhynchia in the Holywell Nodular Chalk above the Plenus Marls Member and, by extrapolation from Dover, the Base of the Fagesia catinus ammonite Zone. The interval up to 5M above the Plenus Marls is rather inaccessible & incompletely exposed. These Beds were assigned to the Melbourn Rock; however, the Upper Limit chosen is probably significantly higher than the top of the Melbourn Rock as identified at Ashwell Quarry, in the single extant locality of the 3 original type Localities, namely Melbourn, Ashwell & Hitchin Railway Cutting Quarry.
The 0.10M Intraclastic Marl, and the 0.05M Flaser Marl overlying Indurated Nodular Chalk, at 5M & 6M above the Plenus Marls Member respectively, are inferred to correlate with the Gun Gardens Marls in the sense of Gale (1996). The Base of the Lower of these 2 Marl Seams was taken to mark the top of the Melbourn Rock. The extremely shell detrital-rich Chalks above & below a Marl Seam 8.3M above the Base are tentatively correlated with the Beds associated with the Gun Gardens Marls. The Filograna avita Horizon, which occurs towards the top of this interval, and is approximately coincident with the most shell-detrital Chalk in the Holywell Nodular Chalk Formation, has not so far been identified at Chinnor, but its inferred position is shown. It is known to be present near the top of the Pitstone Quarry No.2 RIGS Site Section, 24 Km to the North. The latter locality is the most Northerly Section where this important Bio-event is seen in its normal development. The highest Beds at Chinnor Chalk Pit consist of some 2M of shell-detrital Chalks underlain by a Marl Seam. It is possible that this latter Marl Seam is the equivalent of the Aston Marl, which is seen farther to the North in the Ivinghoe-Aston Pit and at the top of the Totternhoe Quarry N0.2a. This Marl Seam marks the lowest Level at which Flint is developed in the Holywell Nodular Chalk in the Central & Northern Chiltern Hills. If this correlation is correct, it suggests that the Top of the preserved Chalk at Chinnor Chalk Pit may be relatively close to the Top of the Holywell Nodular Chalk. However, there is no evidence of Flint at this level in this part of the Chiltern Hills.
The Site exposes one of the most important successions in the Transitional Province, spanning the greater part of the West Melbury Marly Chalk, the Totternhoe Stone (which is particularly fossiliferous here), the Zig Zag Chalk Formation and the Holywell Nodular Chalk Formation. It is one of the most intensively researched Grey Chalk Subgroup Sites in the UK in respect of integrated macrofossil & microfossil biostratigraphy, as well as Stable Isotope Stratigraphy, and it can be used to Link the Transitional Province to the standard Southern Province successions in the Folkestone to Kingsdown & Southerham Grey Pit GCR Sites.