Chalk of Kent

The Chalk of the White Cliffs of Dover; the latest stratigraphy with images. Of Cenomanian,

Turonian, Coniacian, Santonian and Campanian age.


C. S. Harris


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The Chalk Group; Lower Chalk Formation; Glauconitic Marl; Chalk Marl; Grey Chalk; Jukes-Browne Bed 7;

White Bed; Plenus Marls; Middle Chalk Formation; Holywell Nodular Chalk Member; New Pit Chalk Member;

Upper Chalk Formation ; In the Field

Picture of the White Cliffs courtesy of Dover Tourist Board


Part 1. The Theory

The Chalk Group

The regional dip brings the Chalk Group to sea level to the east of Folkestone in East Wear Bay. Chalk occupies the high cliffs from there to Dover and thence as far as Kingsdown, where the cliffs are lower, obscured by vegetation and set back a short distance from the sea. To the north of Deal, the Chalk outcrop is interrupted by the Tertiary deposits of the Richborough Syncline. The Chalk then reappears in the Isle of Thanet, where the highest beds are exposed in relatively low cliffs. Excellent exposures at the base of the cliffs, supplemented by sections provided by cliff paths (Abbot's Cliff, Lydden, Aker's Steps) between Folkestone and Dover, and Langdon Stairs to the east of Dover, permit a composite succession of some 180 m to be logged in detail (Jenkyns et al, 1994, Figs 13a-c).

A useful summary of the Chalk of the Dover and Thanet areas, with a comprehensive bibliography, was given by Shephard-Thorn (1988); the earlier account of the coast sections between Folkestone and Dover (Smart et al, 1966), however, is now considerably out of date. A revision of Peake's (1967) field guide was published by Mortimore (1997). This latter field guide forms the basis for the interpretation of the sections illustrated in this web page.

The Chalk Group is traditionally divided into Lower, Middle and Upper subdivisions, which are effectively of formation status. The top of the Lower Chalk is marked by the Plenus Marls, the base of the Middle Chalk being formed by the Melbourn Rock or by its lateral equivalent. The base of the Upper Chalk in condensed platform and marginal successions is taken at the base of the Chalk Rock, a complex sequence of glauconitised and phosphatised hardgrounds (Bromley and Gale, 1982). In the more basinal successions of Sussex and east Kent, where the Chalk Rock is not developed, this datum was drawn instead at the base of the traditional Sternotaxis plana Zone, which is there marked by a succession including conspicuous, laterally continuous marl seams and large flints, constituting the so-called Basal Complex (Mortimore and Wood, 1986). These basal beds are very conspicuous in the cliff sections east of Dover (e.g. Jukes-Browne and Hill, 1904, Fig. 45) and they consequently attracted the attention of many earlier workers, notably Hill (1886) and Rowe (1899, 1900), as well as that of visiting Chalk stratigraphers from the other side of the Channel such as Hebert (1874) and Barrois (1876)- see Gale and Cleevely (1989) for a useful historical review.

In recent years, the biostratigraphical classification of the Chalk in southern England into a succession of somewhat subjectively defined macrofossil assemblage biozones, which has dominated Chalk stratigraphical philosophy to its detriment for most of this century, has been supplemented by schemes based on lithostratigraphical units and marker horizons, such as marl seams and flint bands. Schemes of this type have been introduced for the Middle and Upper Chalk (the White Chalk of Rowe) by Robinson (1986) and by Mortimore (1983, 1986) for the successions in the North Downs (including east Kent) and Sussex respectively. As shown by Mortimore (1987), Mortimore and Pomerol (1987) and Gale et al (1987), the Mortimore scheme, which was established for the thicker and consequently more complete South Downs successions, is largely applicable to the North Downs as well, with most of the major marker marl seams being recognisable in both areas. In this account, the Mortimore scheme has been used in preference to that of Robinson, except in the case of the highest part of the succession, for which, following the recommendations of Gale et al (1987), it is necessary to employ the term Margate Chalk.

The traditional concept of the Upper Chalk in which the base falls at the base of the Basal Complex and within the Lewes Chalk Member of Mortimore (1986) has been revised by BGS in its current map and memoir programme (Bristow, Mortimore and Wood, in prep.). In this new classification, the base of the Upper Chalk is taken not at the (local) onset of conspicuous nodular flints as before, but at the onset of nodular chalk at the (redefined) base of the Lewes Chalk. A comparison of the old and new classifications, including the main Mortimore subdivisions and the key marker horizons for all the preserved Middle and Upper Chalk of Kent are shown in Fig. 1 below.

Fig. 1 Stratigraphic summary using the revised terminology (Mortimore, 1997) showing some of the equivalent local terminology

Table 1: Stratigraphical summary (using the traditional concept of the M-U Chalk boundary)showing total thickness

Lower Chalk

Middle Chalk

Upper Chalk

Total Thickness of Chalk

78 m

72 m

106 m

256 m

Lower Chalk Formation

The Lower Chalk between Folkestone and Dover is 78 m thick (Jenkyns et al, 1994, Fig. l3a). The classification introduced by the Geological Survey at the turn of the century (Jukes-Browne and Hill, 1903) and still in use today divides the Lower Chalk into a basal Glauconitic Marl, succeeded upwards by the Chalk Marl, Grey Chalk and Plenus Marls. At Dover, Jukes-Browne and Hill (1903) recognised two additional units within the broadly conceived Grey Chalk and below the Plenus Marls: their Bed 7 [usually referred to as Jukes-Browne Bed 71 and Bed 8 or the 'White Bed', in ascending order. Kennedy (1969, Fig. 2) published the first detailed measured sections of the Lower Chalk exposed in the cliffs between Folkestone and Dover and divided the succession beneath the terminal Plenus Marls into 14 numbered beds. Subsequently, better exposures than were available to Kennedy, particularly on a rotated block on the foreshore have allowed detailed measurements of the succession for the first time and have corrected previous misconceptions on the part of all observers regarding the stratigraphy and thickness of the lower part of the Chalk Marl (see Gale, 1989, 1990; Jenkyns et al, 1994).


Glauconitic Marl

The base of the Lower Chalk is marked by the Glauconitic Marl, a unit up to 7 m or more in thickness, comprising dark grey marl with abundant glauconite grains and sporadic phosphatic clasts. The Glauconitic Marl rests non-sequentially and with erosive contact on the Upper Gault, typically on Bed XIII, but locally (e.g. Channel Tunnel Site Investigation, Craelius No. 2 Borehole) on Bed XI or even, in offshore boreholes (e.g. P000), on the undated Zone 6a (Hart, 1993, Fig. 2). The sediment of the Glauconitic Marl is piped down into the underlying Gault in a Thalassinoides burrow system. The unit is intensely bioturbated: the most conspicuous ichnofossils, generally but probably incorrectly referred to Spongeliomorpha, are cylindrical burrows with a central core of marl without conspicuous glauconite. The Glauconitic Marl exhibits poorly developed rhythmicity towards the top and is locally overlain by a thin, weakly glauconitic limestone (Marker horizon M2 of Gale, 1989), which has yielded the zonal index of the Neostlingoceras carcitanense Subzone.


Chalk Marl

Notwithstanding the recommendation of Jukes-Browne and Hill (1903), most workers on the Folkestone Lower Chalk succession (including TML) have recognised a broad and ill-defined subdivision of the interval from the Glauconitic Marl to the Plenus Marls into the Chalk Marl and Grey Chalk. The Chalk Marl is characterised by relatively dark, rhythmically bedded sediments and an overall CaCO3 content below 75%. Each rhythm or couplet typically comprises a basal dark bioclastic marl resting on the burrowed surface of the underlying couplet, above which there is an upward gradation with decreasing clay and increasing carbonate content to a pale cemented, spongiferous limestone (Destombes and Shephard-Thorn, 1971, Fig. 2). In the lowest and highest beds of the Chalk Marl (in the traditional sense, not that of TML), the sedimentary rhythmicity is very conspicuous, but in the middle part of the succession the rhythmicity is rather indistinct due to a higher overall clay content. Near the top of the Chalk Marl as traditionally understood, a closely spaced pair of conspicuous massive, prominent-weathering limestones provides an important marker horizon in the cliffs between Folkestone and Dover. The lower of these limestones overlies a conspicuous dark bed characterised by a 'pulse fauna' including Oxytoma seminudum and 'Chlamys' arlesiensis (Paul et al, 1994). The higher limestone (the so-called Tenuis Limestone from the occurrence of Inoceramus tenuis) underlies the famous 'Cast Bed' of 19th Century fossil collectors, which was so named from the relative abundance of composite moulds of originally aragonite-shelled molluscs, notably gastropods. The Cast Bed yields very rare examples of the belemnite Actinocamax primus and abundant Entolium.

The Cast Bed is followed by several metres of conspicuously rhythmic marl-limestone alternations characterised by Orbirhynchia mantelliana and constituting the Orbirhynchia mantelliana Band as originally described by Kennedy (1969). However, it is now known that this is the topmost of three such Orbirhynchia bands developed in southern England. The (third) Orbirhynchia mantelliana Band terminates in a limestone with a flood abundance of Sciponoceras baculoide, above which there is a sudden shift in the ratio of planktonic to benthonic foraminifera, with an increase of the former over the latter. This shift is termed the PB break and also, because it is coincident with evidence of sedimentary discordance in mid-Channel boreholes (Carter and Destombes, 1972; Hart, 1993; Amédro, 1994), the mid-Cenomanian non-sequence (Carter and Hart, 1977 and references therein). Six further marl-limestone couplets are found above the PB break, which thus falls within, but not at the top of the Chalk Marl.


Grey Chalk

In contrast to the Chalk Marl, the Grey Chalk is characterised by paler coloured, less distinctly rhythmic sediments with an overall CaCO3 content exceeding 75%. A typical couplet begins with a thin flaser marl and terminates in a marly chalk rather than a hard, spongiferous limestone (Destombes and Shephard-Thorn, 1971, Fig. 2.). As in the case of the Chalk Marl, the rhythmicity is inferred to reflect orbital control of productivity. The Grey Chalk is generally much less fossiliferous, although the terebratulid brachiopod Concinnithyris subundata is relatively common near the base and 'Inoceramus atlanticus characterises the highest beds.


Jukes-Browne Bed 7

The Grey Chalk is followed by a group of beds of relatively coarse bioclastic, extensively bioturbated chalks, some 2 m thick, containing large specimens of the zonal index ammonite Acanthoceras jukesbrownei, as well as calcarenite-filled structures (the laminated structures of earlier literature), which tend to stand proud on weathered surfaces, and give a distinctive appearance to the bed.

Although these have been interpreted as scours, current opinion increasingly favours the idea that these structures represent truncated burrow-fills. This bed is traditionally known as Jukes-Browne Bed 7, although Robinson (1986) formally designated it as the Hay Cliff Member. The more many basal part is characterised by a concentration of small oysters (Pycnodonte sp.), which is also found in correlative developments elsewhere.

White Bed (Capel-le-Ferne Member of Robinson, 1986)

The White Bed is a unit restricted to the North Downs and the northeast margin of the Paris Basin (Mortimore et al, 1989) comprising extremely soft, poorly fossiliferous, homogeneous white chalk, with regular transverse and vertical joints. The soft nature of this unit is emphasised by the quarry workers' name 'soapstone'. Few fossils are found apart from sporadic concentrations of the exogyrine oyster Amphidonte sp. and sparse localised occurrences of Inoceramus ictus.


Plenus Marls

The Plenus Mans comprise a thin, clearly defined unit of alternating, relatively fossiliferous, slightly green-coloured marls and marly limestones, which has commonly been given member or even formation status. The base of the Plenus Marls is a major erosion surface and sequence boundary, with Plenus Marls sediments piped down in burrows for up to 0.5 m into the underlying chalk of the White Bed. Based on the work of Jeffries (1963), the succession is divided into eight beds, which have been inferred to represent, together with the basal limestone of the overlying succession, five marl-limestone precession couplets (Gale, 1990; Lamolda et al, 1994). The Plenus Marls takes its name (and the earlier name of Belemnite Marls) from the common occurrence of the belemnite Actinocamax plenus in the higher part of the succession. The Plenus Marls mark the base of a major complex positive 13C excursion, which extends into the basal part of the overlying beds (Gale et al, 1993, Fig. 2) and is commonly referred to as the Oceanic Anoxic Event II. This excursion is accompanied by a significant stepwise extinction of the greater part of the microfauna and microflora, which has been generally, but not universally, attributed to increasing anoxic conditions followed by a gradual recovery as oxic conditions became reestablished. These faunal and geochemical changes have been the subject of intense multidisciplinary investigation in the relatively thin Plenus Marls successions at and near Dover (Jarvis et al, 1988; Leary et al, 1989; Jeans et al, 1991; Lamolda et al, 1994).


Middle Chalk Formation

Holywell Chalk Member

Compared with the Sussex succession, the basal part of the Middle Chalk (uppermost Cenomanian and Lower Turning) in Kent is greatly condensed, forming the so-called Melbourn Rock Beds (Robinson, 1986), the Grit Bed of earlier literature. This part of the succession constitutes the Holywell Chalk of the standard lithostratigraphical classification used by BGS (Bristow et al, in prep.). The topmost Cenomanian (terminal Metoicoceras geslinianum Zone and Neocardioceras juddii Zone), including the two lower pairs of Meads Marls of the Eastbourne succession (Mortimore, 1986; Pomerol and Mortimore, 1993; Gale et al, 1993), is hence represented by about 1 m of intensely hard limestones with Sciponoceras, the marls themselves being recognisable only as thin marl wisps. The base of the Turonian Stage is taken at the base of the succeeding less indurated chalks by extrapolation from the Sussex succession (Gale et al, 1993; Pomerol and Mortimore, 1993). The greater part of the Holywell Chalk is characterised by shell-detrital (predominantly fragmented and comminuted Mytiloides spp.) and intraclastic chalks. Near the base, a bed of calcarenite largely composed of microcrinoid debris (Roveacrinus) yields Fagesia catinus. Higher in the succession, the content of shell detritus reaches a maximum (including a bed with serpulid-encrusted Mytiloides, the so-called Filograna avita bed, which can be traced throughout the Anglo-Paris Basin), above which there is a rhythmic alternation of shell-detrital chalks and non-shelly chalks. By extrapolation from N. American successions, the base of the Middle Turonian can be inferred to lie in the higher part of this rhythmic alternation, above an horizon which has yielded the highest Mammites nodosoides, together with Morrowites wingi and Metasigaloceras rusticum. However, the first unequivocal records of Middle Tunonian ammonites (Collignoniceras woollgari) are from the basal part of the overlying New Pit Chalk in Sussex, i.e. above the upper limit of the shell-detrital chalks of the Holywell Chalk.


New Pit Chalk Member

The Holywell Chalk is succeeded abruptly by relatively poorly fossiliferous, smooth, inconspicuously rhythmically bedded white chalks without shell-detritus. The unit is flintless in east Kent, except at the top, where small flints are locally found within the Glynde Marls sequence. However, to the west, in the vicinity of the Medway, small inconspicuous flints (the Glyndebourne Flints of Sussex) are again developed at the base of the member. Three conspicuous clay-rich marl seams, several centimetres thick (the Round Down Marl, New Pit Marl 1 and New Pit Marl 2, in ascending order) are a feature of the New Pit Chalk in the cliff path sections west of Dover. Close to the top of the member, a closely spaced and laterally variable group of up to 6 marl seams (the Glynde Marls) provides another useful marker; Robinson (1986) introduced the name Maxton Marls for this group to emphasise the difficulty of achieving exact correlations between the individual marl seams in the North Downs succession at this level with those comprising the Glynde Marls of the South Downs.


Upper Chalk Formation

The base of the Upper Chalk is now taken by BGS at the (revised) base of the Lewes Chalk, an horizon which is marked by the onset up-section of nodular chalk, a short distance above the highest of the Glynde/Maxton Marls and at approximately the level of the first flint in the succession (Lydden Spout Flint). It must be emphasised that the base of the Upper Chalk as currently mapped by BGS (Bristow et al, in prep.) is significantly lower than the traditional lower limit at the base of the so-called Basal Complex.

As with the underlying New Pit Chalk, there are several conspicuous marl seams, ranging from less than one, to several centimetres in thickness. All these marl seams produce distinctive spikes in downhole geophysical logs of wells and boreholes (e.g. Mortimore, 1986; Mortimore and Pomerol, 1987) and can thus be more or less readily correlated in the Chalk subcrop. In addition, Wray and Gale (1993) have demonstrated that each marl seam has its own characteristic trace element composition which provides a more or less unique geochemical 'fingerprint'. Rare earth element analysis of the marls by Wray (in prep.) has demonstrated that the New Pit Marls, the higher Glynde Marls, Southerham Marl 2 and Bridgewick Marl 2 have the signature characteristic of a detrital marl, whereas Glynde 1, Southerham 1, Caburn and Bridgewick 1 are distinguished by a significant negative europium anomaly and are thus probably of vulcanogenic origin and comparable to the approximately contemporaneous tuffs in the German Chalk (Wray, 1995).


Part 2. In the Field

The Gault Clay is visible at Copt Point above the Folkestone Beds. The Gault is often slipped and is currently exhibiting numerous mudslides due to the heavy winter rainfall. The basal bed of the Cenomanian is currently freshly exposed directly on the seaward side of the coastguard lookout at the edge of the pitch and putt course, this being the result of recent landslip movement.

The view from the cliff top car park on the Dover side of Copt Point, Folkestone. Showing the landslipped undercliff (Folkestone Warren), the sea defenses and toeweights to the landslips, the 'White Cliffs' between Folkestone and Dover, and Samphire Hoe/Dover Harbour (far right). Horses Head and Abbot's Cliff (far right cliff) are visible.

The Warren is accessible either by walking along the seafront (the favoured route, tide permitting) or down the path adjacent to the cliff top cafe at Capel le Ferne. This latter path is steep and when recently visited showed signs of failure at the top, making it important to take care in the descent. The path leads to a railway bridge from where the cliffs can be observed.

The Horses Head tilted block of Holywell Nodular Chalk. Looking towards Folkestone. Further to the east, at the eastern end of the Groynes, the wavecut platform when exposed yields sections ranging in age from the topmost gault and Glauconitic Marl and basal West Melbury marly Chalk. This section proved important in the interpretation of the Channel Tunnel tunnelling horizon. Here the work of Gale (1989) allowed a correlation across the channel to be established.


The cliffs above Samphire Hoe, the area of reclaimed land created with the spoil from the UK side of the Channel Tunnel. The Folkestone to Dover railway runs along the base of the cliffs. Flinty Chalk is present at the cliff top. Here, although the Aker's Steps are essentially inaccessible now, but the Plenus Marls and the Holywell nodular Chalk are clearly visible in the cliff section. As it is now possible to drive down to this location it is a good starting point for the Chalk of the Folkestone to Dover section.

The cliffs above Samphire Hoe, the area of reclaimed land created with the spoil from the UK side of the Channel Tunnel, looking towards Dover showing the access adit form the cliff top for the Channel Tunnel works and Akers Steps directly to the right and above. The buildings are part of the permanent facilities for the Channel Tunnel.

The view of the White Cliffs from the eastern end of Dover seafront. The nodular chalk of the Lewes Chalk is clearly visible. It is this section eastwards to St. Margaret's Bay that is often considered to be the classic white chalk section.

The view of the White Cliffs from the eastern end of Dover seafront. The western end of this section of cliffline is below Dover Castle.


Fig 2. Section showing the apparent dip of the Chalk in the White Cliffs between Dover Castle and St. Margaret's Bay.

Recent minor rockfall in the White cliffs at the eastern end of St. Margaret's Bay. At least 6 rockfalls, some major (eg. below the lighthouses), occurred during the winter of 2000/01 in the cliffs on either side of St. Margaret's Bay.

The Chalk section viewed from the car park in St. Margaret's Bay.



Engineering Geology of the Channel Tunnel (Harris et. al.)

The Chalk of Sussex and Kent (Mortimore, 1997; G.A. Guide No. 57))







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