The chalk has eaten my homework

Brian John has now put numbers on the claim that the Salisbury Plain we walk on is not the Salisbury Plain an Anglian ice sheet would have stood on. In today’s post the land surface is said to have fallen by 8–10 m over the last half million years. That piece sits on a run of earlier arguments: that the southern glacial limit is farther south than the mapped evidence (February 2022); that Salisbury Plain itself shows no traces of ice action; that sarsens and solution hollows are a long-standing dilemma (December 2016; August 2016); that periglacial processes have done much of the work (January 2023; Murton et al. in the Lyell volume); that Newall’s Mound is a solution hollow inverted into a hummock capped by 1.5 m of clay-with-flints, likened to a kame (28 August); and that the chalk downs are a “serene landscape but lethal geochemistry” in which any till would be destroyed.

The Paleogene silcrete that became the sarsen scatter is given 30–100 m of subsequent lowering, with a conservative 50 m used as a working figure. Quaternary rates are lined up from several places: 15–30 mm per millennium on modern UK chalk, 20–25 mm/ka from the Norber pedestals, 10–15 mm/ka in the Driftless Area, ~20 mm/ka on the Normandy chalk plateau. The conclusion offered is that “the landscape as it was at the time of the Anglian Glaciation … is simply not there any more. End of story.”

The rates are in the right order of magnitude. Chalk does dissolve. Solution pipes, coombe rock, head and clay-with-flints have been mapped across Wessex for more than a century. Jukes-Browne already knew that pure chalk does not contain enough insoluble residue to make the plateau clays we see. None of that is new, and none of it requires a Pleistocene ice sheet on the Wiltshire downs.

What the new numbers do not do is make a till sheet disappear without leaving the fraction that chalk chemistry cannot touch.

Alkaline water does not eat silicates

White Chalk is often more than 98% CaCO₃. Meteoric water charged with carbonic acid removes that matrix. The porewaters and rendzina soils that result are alkaline to circumneutral, buffered by excess carbonate. They are not an acid bath. Quartz, zircon, rutile, tourmaline and dense igneous clasts do not dissolve on Quaternary timescales. Frost shatters a dolerite. Periglacial attrition turns it into gravel, sand and heavy-mineral grains. It does not vaporise the silicate cargo into thin air.

Eight or ten metres of surface lowering on a carbonate plateau concentrates that residue; it does not erase it. Clay-with-flints is exactly that sort of remanié — chalk gone, insolubles left, later mixed with Palaeogene cover and cryoturbated. John now says as much of Newall’s Mound: an old hollow whose flanks have lowered while the fill stayed put, so the sink becomes a low hummock under 1.5 m of clay-with-flints. That is ordinary downland inversion. It is not a vanished Irish Sea till sheet, and dressing it as a kame analogue is a mountain made from a molehill. If ice had dumped even a thin and patchy till, the same logic still applies. After the carbonate matrix had gone you would still expect quartz, zircon, rutile, tourmaline, micro-lithics of dolerite and rhyolite, and far-travelled pebbles in residual soils, in those inverted caps, and in the river gravels that drain the Plain.

That test has been run.

The grains are local

Stream sands draining Salisbury Plain carry detrital zircon suites whose age spectra match recycled southern British Mesozoic and Palaeogene sources — the London Basin / Thanet Formation pattern — not a first-cycle dump from the Lower Palaeozoic of Wales or a crystalline northern shield. In the 2026 U–Pb study, 550 zircon analyses produced a single concordant grain at 464 ± 16 Ma, the Darriwilian age that characterises the Mynydd Preseli / Fishguard Volcanic Group sources of the Stonehenge rhyolites. One grain in hundreds is the opposite of the Welsh igneous signal required if ice had carried dozens of multi-tonne bluestones and ground the rest into rock flour. The apatite record is dominated by a Cenozoic reset (~60 Ma), not Palaeozoic basement.

Clast counts in the Wiltshire Avon terraces are the same story at pebble scale: local flint, sarsen and Greensand chert, not the exotic suite an Irish Sea or Welsh ice stream would impose. Field survey still fails to produce a bluestone erratic in an undisturbed Quaternary context away from the monument and its debitage. The Newall boulder matches Craig Rhos-y-felin rhyolite of the Stone 32d group and was moved by people. Coastal patches in North Devon or Somerset do not put a grounded lobe on the Wiltshire chalk plateau.

If 8–10 m of the old land surface has gone, those minerals and clasts are what should have been concentrated into the modern drainage. They are present. They do not show a Welsh ice sheet.

Models are not deposits

Numerical ice-sheet models are not a substitute for sediment on the ground. A model can be made to grow ice almost anywhere if the climate forcing, sliding law and bed friction are generous enough. That is why older, lightly constrained experiments are still cited as showing it was “perfectly possible” for ice to reach east of the chalk scarp. Possibility in a sensitivity run is not a reconstruction.

The reconstructions actually tied to mapped margins and dates — BRITICE-CHRONO and the later ensemble tests against flow geometry and deglaciation timing — put Devensian Irish Sea ice to the Celtic Sea shelf edge and onto parts of the north Devon and Somerset coasts. They do not put a grounded lobe over the Wiltshire chalk plateau. The Anglian ice sheet, the one usually invoked for a 450-ka till on Salisbury Plain, is even less a creature of those high-resolution Devensian models. Its limit of preserved evidence remains well north of Wessex.

And even if a future ensemble were forced across the downs, the prediction would still be a first-cycle Welsh and Irish Sea mineral load in the Avon sands and terrace gravels. That load is not there. A model that contradicts the grains is a discarded run, not a hidden ice sheet.

The loop has a new rung

The argument has retreated through a closed set of auxiliary hypotheses:

1. Missing moraines and till → cold-based ice.
2. Need large megaliths moved → warm-based basal plucking.
3. Missing gravel-sized erratics across the Plain → corrosive chalk chemistry (27 August).
4. Why the bluestones survive at one later monument → left unexplained.
5. New: the land surface itself has been lowered by 8–10 m (29 August), so the original deposits “must long since have disappeared.”

Each failed prediction is reclassified as a feature of the model. The fifth step is the same move as the third, only taller. Surface lowering is not in dispute. What it does not licence is the claim that durable silicates vanished with the carbonate. After 150 years of stratigraphy, terrace clast surveys and grain-scale U–Pb geochronology, the record across Salisbury Plain is not silent. It is negative.