The death of the Upper Greensand: A stratigraphic obituary for a vanished formation
Jon Trevelyan (UK)
For much of the twentieth century, generations of students (and amateurs like me) learned a tidy, mid-Cretaceous succession for southern England:
Lower Greensand – Gault Clay – Upper Greensand – Chalk.
It appeared in every textbook, diagram and museum panel. There was an appealing symmetry to the sequence, with two sands bracketing the impervious Gault and the Chalk resting neatly above. It seemed so natural that few doubted it.
Yet modern geological maps of Kent, Surrey and East Sussex tell a different story. The Upper Greensand, once entrenched in the succession, has disappeared entirely from the southeast. In its place lies only the upper Gault Formation, grading seamlessly into the Chalk Marl. The rocks themselves have not changed, but our understanding has.
This is the life and quiet death of a formation: how it rose to prominence, how it lingered long after its foundations had eroded, and how it was finally recognised as a misinterpretation of a more subtle geological reality.
A formation created by symmetry
The Upper Greensand was established during the heroic era of Victorian mapping. In the West Country, where the unit is thick, sandy, glauconitic and unmistakably arenaceous, it made perfect sense. Escarpments near Shaftesbury, chert beds in the Blackdown Hills, and the “Firestone” quarries of the Isle of Wight gave it a clear identity. Jukes-Browne, Whitaker and their contemporaries formalised it confidently.
But as mapping extended eastwards into Surrey and the Weald, the rocks became uncooperative. The unit thinned dramatically. The sandiness weakened. The greenish tint faded. Memoirs disagreed not just about where to draw the boundary, but whether a boundary could be drawn at all. The “Chloritic Marl”, later recognised as part of the Gault-Chalk transition, caused repeated confusion. Its position in the succession changed depending on the author and county.
But still, the Upper Greensand survived. It made the stratigraphic diagrams look balanced, and its presence in the west gave it legitimacy. Victorian geologists valued neatness – the rocks, it turned out, did not (Fig. 1).
The illusion begins to crumble
The supposed Upper Greensand of the southeast rarely resembled its western counterpart. Instead of a stratified sandstone, it was often a pale, chalk-flecked marl, only weakly gritty and only faintly tinted green by scattered glauconite grains. Its gradation from the Gault below was so smooth that, even standing before it, one could scarcely predict where a lithological boundary ought to go.
Victorian geologists could not have known that glauconite forms in many settings – in sands, silts and even marls – wherever sedimentation is slow and porewaters remain oxygenated, and that its green colour signals environmental conditions rather than a distinctive rock type. To them, greenish meant greensand. To modern eyes, that colour is an environmental indicator rather than a definition of rock type.
The marls of Kent masqueraded convincingly as “Upper Greensand” only because the traditional scheme demanded something sandy here. The lithology itself never insisted on the distinction (Fig. 2).
Biostratigraphy wins
The decisive evidence arrived in the mid-twentieth century, with the rise of precise ammonite zonation and the systematic study of microfossils. When Henry Owen and others applied biostratigraphy to the upper Gault-Chalk transition, the result was unambiguous. The “Upper Greensand” of Kent and Sussex fell entirely within late Albian ammonite zones, especially Mortoniceras inflatum. The fossils did not recognise a change. No new fauna appeared; no time break separated the supposed “Upper Greensand” from the Gault below.
A formation that cannot be pinned down in the rocks or recognised by its fossils begins to look less like a unit of stratigraphy and more like a phantom.
A scientific deepening: ammonites, facies and a drowning shelf
As evidence accumulated, it became clear that the mid-Cretaceous of southern England recorded a broad, progressively drowning shelf, deepening from west to east. Nearshore sand continued to reach the western counties long after it had ceased to arrive in Kent. As the shoreline retreated westward and sea levels rose, carbonate deposition began to dominate. Silty, clay-rich Gault sediments gave way gradually to calcareous marls and ultimately to the coccolith-rich Chalk.
This west-east gradient creates a classic diachronous facies belt. A sandy, glauconitic formation in the west becomes a silty glauconitic marl in Surrey, and a chalky marl in Kent. And all broadly time-equivalent but deposited in different environments.
The Upper Greensand, in other words, is a western facies, not a universal formation (Fig. 3).
The rocks of the southeast did not “lose” the Upper Greensand. They were never part of that depositional system to begin with. A west-east cross-section, makes this visually explicit: the robust sands of Devon diminish steadily eastward, weaken into transitional beds in Hampshire, and vanish entirely by the time you reach Kent, replaced by calcareous upper Gault.
| Lithostratigraphy and facies: what’s the difference? |
|---|
| Lithostratigraphy groups rocks by what they are — their composition, appearance and mappability – defining formations as distinctive rock units. Facies describe how those rocks formed, reflecting environment, water depth and sediment supply. Rocks of the same age may form different facies in different settings, and a formation may change laterally as conditions vary. Confusion arises when a facies is mistaken for a formation everywhere: the Upper Greensand is a true formation in the west, but eastwards it grades into marly upper Gault facies rather than persisting as a separate unit. |
Field confirmation from East Wear Bay
The cliffs of East Wear Bay, Folkestone, showcase the transition better than any diagram. Figs. 4 and 5 reveal the characteristic pairing: bluish, blocky Gault Clay below; above it, pale, chalk-rich marls with only the faintest hint of grit. Between the two is no boundary: no erosional surface, no sand body, no change abrupt enough to warrant a formation name, only the slow shift in colour and composition that marks the approach of the Chalk Marl.
This is precisely where older memoirs insisted the Upper Greensand must lie. But the rocks refuse to cooperate. The supposed formation dissolves before one’s eyes.
A survivor in the west
It is important to be clear: the Upper Greensand survives fully and legitimately in the West Country. There, it is thick, distinctive and deservedly recognised as a formation. Fig. 6 represents a distribution map that shows how tightly its occurrences cluster west of the Hampshire Basin and how sharply it disappears eastwards.
So, its survival is not sentimental. It reflects genuine geological reality – a difference in palaeogeography and sediment supply, not a difference in nomenclature.
| What about Dorset and the Isle of Wight? |
|---|
| The disappearance of the Upper Greensand from southeast England does not apply universally. In Dorset, east Devon and the Isle of Wight, the Upper Greensand remains a thick, sandy and genuinely distinctive unit, forming prominent escarpments and supplying building stones such as the historic “Firestone”. These western successions received a continued influx of nearshore sand during the late Albian, even as more easterly areas passed into quieter, deeper-water conditions. The result is a classic west-east facies contrast: a true arenaceous Upper Greensand in the west, grading eastwards into silty and then chalky marls that belong to the upper Gault. The difference reflects palaeogeography and sediment supply, not inconsistency in modern stratigraphy. |
The philosophical turning point
The disappearance of the Upper Greensand from the southeast is part of a broader intellectual shift in geology. Nineteenth-century stratigraphy was largely descriptive: formations were defined by how they looked. Modern stratigraphy integrates lithology, facies, fossil content and depositional context. Boundaries are interpreted, not merely observed.
By these standards, the Upper Greensand in the southeast could not survive. It lacked the continuity, distinctiveness and geological meaning required of a formation. Its removal was not an act of destruction but an act of clarification.
The modern view
In contemporary mapping, the sequence is far simpler:
Lower Greensand Group → Gault Formation (including the marly upper beds) → Chalk Marl (basal Chalk Subgroup) → Grey Chalk → White Chalk.
The traditional four-part arrangement has been retired. The Upper Greensand has been absorbed entirely into the upper Gault in Kent and Sussex, and recognised as a lateral equivalent – not an overlying unit – of the western sands.
Correlation diagrams (Fig. 7) make clear that the Upper Greensand of the west is time-equivalent, but not stratigraphically overlying, the marly upper Gault of the east. What used to appear as an east-west variation in the thickness of a formation is now seen as the expression of a facies shift along a marine shelf.
Lessons from the North Downs
The North Downs offer numerous exposures where the old boundary used to be mapped. At Wrotham, Boxley and the Folkestone cliffs, the transition from Gault to Chalk is more of a blur than a line. The marly upper Gault contains scattered glauconite, thin silty partings, and increasing carbonate; the Chalk Marl begins with softly laminated, coccolith-rich muds. But nothing in these outcrops resembles a discrete greensand. The boundary exists only in the mind of the mapper, not in the cliff.
As field evidence accumulated, the Upper Greensand simply slipped from use. It did not collapse in argument; it faded away in practice.
An epitaph for the Upper Greensand for Southeast England
Born of symmetry, sustained by tradition, undone by evidence, it has not been “abolished”, because you cannot abolish what the rocks do not support. It has simply been recognised for what it always was – a misapplied name, imposed on sediments that never matched the classic facies of the West Country. But its removal does not diminish the beauty of the Cretaceous record in the southeast. It clarifies it.
Further reading
Gale, A.S. (2019). The Cretaceous World. Princeton University Press.
Hopson, P.M. (2005). A Stratigraphical Framework for the Lower Cretaceous of England. BGS RR/05/01.
Jukes-Browne, A.J. (1900–1904). The Cretaceous Rocks of Britain. Geological Survey Memoirs.
Owen, H.G. (2012). “The Gault and Upper Greensand of southern England” Geology Today, 28, 87–94.
Owen, H.G. (1971). “The stratigraphy of the Gault and Upper Greensand of the Weald.” Proc. Geol. Assoc., 82, 593–634.
Rawson, P.F., Allen, P. & Gale, A. (2001). The Chalk Group – Revised Lithostratigraphy. BGS RR/01/03.
Whitaker, W. (1872). The Geology of the London Basin. Geological Survey.
Wood, A. (2010). Geology of the Dorset Coast. Dunedin.