Geologists have several hierarchical classifications which they use to group rocks of similar age or type. A “Period” is a term used for one of the age hierarchies. The bedrock encountered along the Royal Saxon Way (RSW) dates from the Cretaceous Period in the south, through to the Paleogene Period in the north but only some of the rocks from the Paleogene are present, the oldest and youngest are missing and those which remain date from about 59 to 48 million years ago. The majority of the route is underlain by rocks from the Cretaceous Period which date from 126 to 66 million years ago. The bedrock is covered by scattered, younger “Superficial” deposits throughout the route.
During the Cretaceous Period, the shapes of the land masses were nothing like the present; the outline of the current continents had not been established, the future American continent had not yet separated from the current European and African continents. That part of the Earth’s crust which is now the British Isles was covered by sea.
The oldest rocks found along the RSW, the Cretaceous rocks, are sea bed deposits and the sedimentary sequence in south east England reflects the change in sedimentation caused by the enlargement of the Cretaceous sea and the increasing remoteness of land. As time passed in the Lower Cretaceous Period, the distance from the shore increased, so only that finer sediment, which could remain in suspension long enough to be transported any significant distance from land, was deposited – mainly silts and clays. In the Upper Cretaceous Period the sea had enlarged to an extent where the majority of it was too far from land to receive any land derived sediment leaving only marine debris to accumulate on the sea bed.
The following notes are based on information supplied in the British Geological Survey’s on-line resources, Lexicon of Named Rock Units, Geology of Britain Viewer and Geo Index Onshore. Reference was also made to the 1:50,000 scale printed map sheet 305 & 306 Folkestone and Dover, and Sheet 289 Canterbury, together with the corresponding memoir for those sheets, “Geology of the country around Canterbury and Folkestone” published by the Geological Survey of Great Britain “The Memoir” (Smart et al., 1966).
The online index and mapping is described as “Dynamic” by the BGS – the descriptions and classifications are subject to change. The bulk of the information below was accessed in 2022, but more recent alterations are signified when noticed.
Starting from the oldest in the south and working to the youngest in the north, the rock sequence along the RSW is as follows:
The Lower Greensand Group (Lower Cretaceous Epoch).
The Atherfield Clay Formation.
Age: 126 – 113 million years
Thickness: About 13m in boreholes in Folkestone and Hythe.
This is oldest geological deposit that you will walk over along the RSW. It was deposited at a time when this area was a delta which was slowly being inundated by the sea from the south. It is rarely seen in exposures as it is thin and usually covered by the overlying Hythe Formation which has slipped downslope and other slope debris. In its unweathered state, the Atherfield Clay is a bluish grey, slightly sandy, clay or mudstone (mudstone is a hard clay). In the Sandgate area the base has a distinctive chocolate brown colour.
The Hythe Formation.
Age: Between 126 – 113 million years.
Thickness: About 7m – 10m in boreholes in Folkestone.
This comprises alternating layers of limestone (“Ragstone”) and calcareous greenish sand (“Hassock”) each layer being between 0.1m and 0.3m thick in east Kent. The top is marked by an unconformity (an abrupt change in the composition and age of the deposit). It can be seen inland in small cliffs along the Greensand Escarpment and in quarries. It has been extensively quarried further west, particularly around Maidstone where the where the limestone is thicker. It was used to supply building stone which you can see in many old houses and walls in Kent. The limestone is unusually dense, more dense than concrete. At the time that the Hythe Formation was being deposited the preceding delta had become submerged by a shallow sea. The limestones are the remains of shell debris and coral reefs and the sands are the result of periodic incursions of land derived sediments in shallower seas.
The Sandgate Formation.
Age: Between 126 – 113 million years.
Thickness: About 36m in the Metropole Hotel borehole.
This is rarely exposed but comprises fine sands, silts and silty clays, often green coloured, some iron stained sands and some weak sandstones. In the Sandgate area the base is a dark grey-black sandy clay. The Sandgate Formation was deposited in shallow seas but further from land than the reefs of the Hythe Formation, so only the finer grained land derived material was able to reach this distance from the shore.
Most of the Sandgate Formation is not in situ but is land-slipped material which commonly includes decaying plant debris. Large rafts of apparently intact Sandgate clay several metres across can be found overlying land slip debris with decaying organic material (personal experience and (Topley, W. (1803)).
The Folkestone Formation.
Age: Between 126 and 101 million years.
Thickness: About 30m – 43m in the escarpment from Ashford to Sandgate.
This comprises medium to coarse grained, white, yellow or red brown (iron rich) sands and sandstones. East of Stanford, the top includes a calcareous sandstone, known locally as Folkestone Stone, which includes characteristic coarse sub rounded quartz grains. This sandstone produces a secondary escarpment. Ordnance Survey maps, since the late 19th century, show several ‘Sand Pits’ along this escarpment between Sandling and Newington. There is an extensive exposure of this sandstone in a quarried face beyond the east boundary of the new pitch at Folkestone Rugby Club (TR 1749 3688, ///drumbeat.about.protect). The local geological memoir, Smart et al., (1966), mentions the presence of ironstone concretions in several quarry exposures.
Selborne Group (Lower Cretaceous Epoch).
The Gault Formation
Age: Between 113 to 101 million years.
Thickness: About 30m – 40m at Ottinge borehole and Folkestone Warren respectively. Thinning northward.
This is a grey to dark grey mudstone weathering to clay which contains thin bands of nodules sometimes containing fossils. It was deposited in a marine environment at a time when the sea bed was becoming deeper and more distant from land so that only the fine, suspended solids were available to form the deposit. The Gault Formation also marks the top of the Lower Cretaceous Series.
The Chalk Group (The Upper Cretaceous Epoch)
Age 101 to 66 million years ago.
The total thickness of the Chalk varies according to how much was originally deposited in the various basins and how much has been subsequently eroded. It varies from 200m to 560m within its outcrop but may be up to 1km below the North Sea. In the UK it’s outcrop extends from Dorset, via Norfolk, up to Flanborough Head in North Yorkshire and east into Sussex and Kent.
This deposit marks the end of sedimentation derived from the land and the beginning of sediments derived from a marine origin as the chalk sea enlarged and land became more distant. A “Hothouse” climate existed at this time. As a result, the chalk sea was host to an almost permanent bloom of algae called Coccolithophores which contained microscopic plates of calcium carbonate (coccoliths). When these algae died, the coccoliths settled onto the sea bed and formed a calcium rich silt which, after hundreds of thousands of years of compaction, solidified to form the Chalk.
Most of the Chalk is almost pure calcium carbonate, but the earliest sediment (The Grey Chalk) was deposited when the chalk sea was within the influence of land derived sediment. This was a fine grained clayey, material which produced a grey to off-white, clayey chalk (Marl). Most of the Chalk, in southern England at least, is a white mostly very weak limestone (The White Chalk) but it includes hard beds. Other impurities in the White Chalk are flints (see “How flints in chalk were formed” for more information) and thin marl seams which in some cases have been shown to be derived from volcanic ash.
The Chalk Group is divided into a lower Grey Chalk Sub-Group which comprises two Formations, and the overlaying White Chalk Sub-Group which comprises seven Formations in total although the top two are missing in the south east.
The Grey Chalk Sub-Group.
Age: 101 – 94 million years.
This Chalk is characterised by having a clay content which gives it a light grey to buff colour and a lower permeability than the White Chalk. The clay content is land derived but a very fine material which could be held in suspension in the sea for long enough to be transported beyond the normal influence of the land. Chalk containing clay is referred to as Marl.
This Subgroup comprises two Formations:
West Melbury Marly Chalk Formation
Thickness: About 15m – 25m.
Alternating layers of grey, sparsely glauconitic chalky marl or marly chalk, and occasionally hard grey limestone. The base usually includes the Glauconitic Marl Member which is a calcareous glauconitic sand and glauconitic sandy silty chalk with phosphatic nodules which is up to 5m thick in Kent.
Zig Zag Chalk Formation (named after Zig Zag Hill near Shaftsbury).
Thickness: About 35m -50m.
Mostly firm, pale grey to off-white blocky chalk with alternating layers of marls and marly chalks with firm white chalk near the base. Thin gritty, silty chalk beds act as markers in the sequence.
The White Chalk Sub-Group.
Age: 94 – 66 million years but the top part from 72 – 66 million years is missing in east Kent.
This Chalk is almost pure Calcium Carbonate. Marl seams exist but are relatively thin, usually a few centimetres. This forms much of the iconic, vertical White Cliffs of Dover, Beachy Head in Sussex and the cliffs of the Pas de Calais.
This Subgroup comprises five Formations:
Holywell Nodular Chalk Formation
Thickness: About 25m -35m.
Generally hard nodular chalks with thin wavy marls and abundant shell debris. The base is marked by The Plenus Marls Member. The hard blocky chalk comprising the Melbourn Rock Member is just above the base.
New Pit Chalk Formation
Thickness: About 35m to 50m.
Mostly blocky, white firm to moderately hard chalk with numerous marls seams. Flint occurs sporadically near the top.
Lewes Nodular Chalk Formation
Thickness: about 35m.
Hard to very hard nodular chalks (too hard to scratch with finger-nail) with interbedded soft to medium hard chalks and marls. The nodular chalks are typically lumpy and iron-stained brown and probably derive from sponges. The first regular seams of nodular flint, some large, appear in this formation. In the North Downs, the lowest part of the formation comprises a series of closely spaced marl seams and large nodular flints often called the “Basal Complex”. The Lewes Nodular Chalk has been interpreted as reworked chalk debris resulting from sea bed folding, compression and slides (Mortimore and Pomerol, 1997).
(The Chalk Rock Member (0.5m – 5m thick) is often referred to but is absent in east Kent. Here it is replaced by a series of hard grounds and flints sometimes called The Basal Complex at the base of the Lewis Nodular Chalk Formation).
Seaford Chalk Formation
Thickness: 55m to 60m.
White blocky chalk with conspicuous semi-continuous nodular and tabular flint seams.
Margate Chalk Member (a local variation of the Newhaven Chalk Formation)
Thickness: Up to 24m in north Kent.
Marl-free smooth white chalk with less flint than the Seaford Formation. Some brown iron stained beds derived from sponges. This is a locally developed chalk which is shown on the geological map north east of a line from Canterbury to Dover. It may extend into Essex. Elsewhere it is called the Newhaven Chalk Formation.
The top of the Margate Chalk Member is an erosion surface on which the much younger Palaeogene rocks were deposited.
About 15 million years after the Chalk was deposited, the chalk sea bed was uplifted and planed off to flat surface by a subsequent incursion by the sea, which started the deposition of the Cenozoic sequence.
Montrose Group. (Paleocene Epoch)
Thanet Formation. (This is the only representative of the Montrose Group in southern Britain which is more fully developed in the North Sea area).
Age: 59 – 56 million years.
Thickness: About 30m between Stodmarsh and Wingham.
Pale grey-brown, fine-grained sand and green – grey clay. Distinctive greenish black nodular flints occur at the base called the Bullhead Bed. This is a shallow marine deposit laid down by an incursion of the sea forming an erosion surface over the older Mesozoic rocks.
Lambeth Group. (Paleocene – Eocene Epoch.)
Age: 59 – 48 million years
The Lambeth Group was deposited as uplift caused the sea to retreat to the north east leaving a brackish delta. As a result, these sediments are much more variable and include gravel and even lignite originating from plant debris.
The individual Formations are not defined on the digital map.
Upnor Formation
Thickness: May not be present or if it is, less than 2m.
Typically variable green fine to coarse-grained sand with variable clay and silt content, and with beds and lenses of black flint gravel.
Woolwich Formation
Thickness: Less than 11 – 12m in north Kent, typically 5m near Wingham.
A brackish water deposit, of coarse grained alternate beds of light greyish brown and brown sands, silts and clays giving a striped appearance. There is often a nodular ironstone bed, about 1m thick near the top.
Thames Group. (Paleogene Period, Eocene Epoch)
Age: 56 – 48 million years.
Harwich Formation, Oldhaven Member
Thickness: Less than 7m.
The Harwich Formation was laid down in a gradually deepening sea as the previous retreat was reversed.
It is a marine deposit, typically comprises greenish fine grained bedded sands and silts with thin clay beds. There is a black flint pebble bed at the base. Volcanic ash is a significant minor component in north Kent. The Harwich Formation commonly includes a marine shell deposit.
London Clay Formation
Thickness: up to 150m.
The London Clay Formation, was deposited on a deeper sea bed which was by that time some distance from land where only suspended solids were the source of sediment.
It is a blue-grey weathering to brown, slightly calcareous, silty to very silty clay. It commonly contains thin courses of carbonate concretions (“cementstone nodules, or claystone”). It is sandy towards the top and bottom. Thin beds of black rounded flint gravel common at the base.
Clay-with-flints Formation.
Revised 06/08/2025
Age: 23 – 3 million years.
Thickness: Up to 10m – Variable. Where found on Chalk, it can locally reach 30m – 50m in dissolution features or “Pipes”, elsewhere it can be too thin to map.
Clay-with-flints is a residual weathering deposit, which in this part of Kent, is formed from the dissolution of calcium from the Chalk and the overlying Paleogene deposits.
As its name suggests it is a clay with abundant nodules of flint. It originates in pre-Ice Age times, when the climate was cooling from tropical to warm temperate. The high temperatures promoted chemical weathering which is predominant in warm climates today.
It is typically an orange-brown and red-brown sandy clay with abundant nodules and rounded pebbles of flint. Angular flints are derived from the Chalk, and rounded flints, sand and clay from overlying Palaeogene formations. There is commonly a discontinuous basal layer up to 10 cm thick, with dark brown to black matrix of, stiff, waxy and fissured, clay with relatively fresh flint nodules stained black or dark green with manganese or glauconite.
The BGS has identified a sandy variety of Clay-with-flints which they refer to as ‘Sand in Clay With Flints’. It is found at scattered localities along the crest of the North Downs escarpment between Hastingleigh and Capel-le-Ferne. It is described as a sand-rich unit within reddish brown clay or sandy clay with abundant flint pebbles and rare sarsen sandstone. It also includes coarse gravel sized fragments of iron rich sandstone (Harry Blows personal communication). The sarson sandstone (silcrete) is another residual deposit produced by chemical weathering. It is a silica cemented sandstone, rather than the more usual calcite cemented variety, and is therefore unusually durable. It may be more familiar as Sarsen Stone found on the Wiltshire Downs.
This sand was previously considered to be a variety of Lenham, Beds found on the top of the downs further west, above Lenham, but that is a marine deposit, not a residual deposit. (The BGS has now given the Lenham Beds Formation status and the Sand In Clay With Flints is indexed within the Clay-with-flints Formation).
(N.B. The sandy variety is identified on the sheet mapping with both its own symbol and colour on the map. It is not identified on the online mapping with its own colour, but clicking on the map to obtain further information does reveal the two different deposits.)
Clay-with-flints can be mapped as “Head” or even “River Terrace Deposits” when it is has moved downslope (see below).
River Terrace Deposits
Age: Less than 3 million years.
Thickness: variable over short distances because of incision into underlying beds, 0.5m – 6m.
They are deposited by rivers and vary from coarse to fine-grained gravels and sands, silts and clays. Bedding is often absent, indicative of deposition by high volumes of rapid flowing water (“Torrent Bedded”) or, where there is bedding, it shows distortion typical of freeze-thaw action. In the lower Great Stour Valley, they are notable for the inclusion of large intact rafts of clay, which have been deposited in an upended or completely overturned orientation within the surrounding granular soils. These occurrences have been used as evidence of torrential river flows carrying blocks of frozen ground following rapid thawing. (See the section on How the Elham Valley was formed for further details.) Palaeolithic flint implements are also found, again notably in the Sturry area.
The River Terrace Deposits form beds and lenses in channels and floodplains. They are numbered 5 to 2, each corresponding to depositional phase of the associated river (Nail Bourne or Little Stour in this area). The subsequent downward erosional phase left discrete remnants of gravel perched on the sides of the valley. The 5th terrace is the oldest and furthest above the valley floor and the 2nd is the youngest and closest to the valley floor.
They have been traditionally identified by their elevation (in feet) rather than by the characteristics of the deposit which, being so variable, is unreliable. Recently the use of elevation as a means of correlating deposits has been questioned.
The geological maps show the Fifth to Second Terrace Deposits to be present along the Elham Valley downstream of Barham. For further details and the significance of these deposits see the following section “The origin of the Elham Valley”.
Aldiss & Farrant (2002) advise that “River Terrace Deposits” can merge into “Head” (see below).
Head / Head 1
Age: Post last glaciation.
Head is “catch-all” term used to include, in east Kent at least, fine grained rock debris and/or clayey hillwash and soil creep which accumulates at the foot of slopes. The qualifying feature required for a deposit to be called Head is this down slope movement or “soil creep” (but see Loess below). This can be initiated by two types of Mass movement, solifluction or gelifluction. Solifluction is the slow downslope movement of waterlogged soil and other superficial deposits. The term gelifluction is restricted to the same process where movement is over frozen ground and occurs during the thawing of seasonally frozen ground.
Head is a variable deposit comprising gravel, sand and clay depending on the nature of the upslope source and distance from the source. It can include localised lenses of silt, clay or peat and organic material. Sandy varieties are sometimes called Brickearth and gravelly types are sometimes called Head Gravel.
The BGS distinguish “Head 1”, from the generic term “Head” to indicate an older, more coarser deposit comprising sands and gravels, locally with lenses of silt, clay and peat. It is older than “Head” if it is included in the same mapping area.
In east Kent there is also a silt variant which is wind-blown dust (Loess) accumulated in Tundra desert conditions. This type hasn’t necessarily been subject to downslope creep movements. The BGS now calls them Fluvio-aeolian deposits.
Alluvium (including Dry Valley and Nailbourne Deposits).
Age: Post last glaciation.
Alluvium is a general term for unconsolidated (soft and compressible) detrital material comprising clay, silt, sand and gravel, deposited by a rivers, streams or other bodies of running water, as sediment in the bed of the stream or on its floodplain. A stronger, desiccated clay zone may be present at the surface in floodplains.
Its thickness is variable, especially over the chalk surfaces which include “dissolution features” – deep holes and pipes in the chalk. According to the Memoir (Smart et al., 1966) at one excavation for a cesspit in the valley north of Elham the “Dry Valley and Nailbourne Deposits” were found to be in excess of 4.3m thick and in a similar excavation in Lyminge they were found to be in excess of 3.6m thick and neither excavation reached the Chalk. These are only two datum points along the buried surface of the Chalk which is notoriously variable in depth and so should be treated with caution as an indicator of typical thickness. Nevertheless, the Memoir also notes that generally that there is “much gravel below the Alluvium of the Little Stour River” (Nailbourne) and mentions that gravel was exposed for 4ft (1.2m) in an excavation near the church at Bishopsbourne and 3ft (0.9m) were noted below 3ft of loam in an excavation near Patrixbourne Church.
Tidal Flats Deposits
Age: Post last glaciation.
Tidal flat deposits, including mud flat and sand flat deposits, are deposited on extensive nearly horizontal marshy land in the intertidal zone that is alternately covered and uncovered by the rise and fall of the tide. They consist of unconsolidated sediment, mainly mud and/or sand. Normally a consolidated soft silty clay, with layers of sand, gravel and peat. Characteristically low relief.
Andrew Coleman
Rev. 25/02/2023
References:
Farrant, A. R., & Aldiss, D. T. (2002). A geological model of the North Downs of Kent: the River Medway to the River Great Stour. www.thebgs.co.uk
Smart, J. G. O., Bisson, G., & Worssam, B. C. (1966). Geology of the Country around Canterbury and Folkestone. Her Majesty’s Stationery Office.
Geomusings 