Morphogenesis of Cup-Shaped Depressions in Sarsen Stones

 

The entrance to the Long Barrow at All Cannings, showing natural cup shaped voids in the sarsens.

Sarsen stones (Palaeogene silcretes) are intensely indurated, composed almost entirely of quartz sand grains bound by syntaxial quartz overgrowths. Despite their extreme hardness and chemical stability, their surfaces frequently exhibit circular or sub-circular, cup-shaped depressions. Distinguishing natural geological features from anthropogenic modification (e.g. Neolithic cup marks) requires analysis of a feature's micro-morphology, formational context, and weathering history.

Why the boundaries are sharp

A sarsen boulder typically goes from fully indurated rock to friable or loose sand with little sign of a graded 90%/70%/50%-cemented halo in between — true both at a boulder's outer surface and at internal boundaries against inclusions. This is a property of how silica cementation works, not a gap in the rock record, and it underpins several of the mechanisms described below.

1. Precipitation happens at interfaces, not through a volume. Silica tends to come out of solution where conditions change abruptly — at the water table, at permeability boundaries, where groundwaters of different chemistry mix, or across redox/pH fronts. Once supersaturation is crossed, cementation proceeds rapidly along that interface rather than diffusing evenly outward. This is documented directly in an analogous deposit: tightly cemented sandstone lenses in the Fontainebleau Sand (Oligocene, Paris Basin) sit immediately within otherwise loose, unconsolidated sand, with the sharp contrast attributed to silica precipitating along a specific hydrological interface (Thiry & Maréchal, 2001).
2. The cement grows in discrete pulses, not a steady film. Cathodoluminescence imaging of Stonehenge sarsen Stone 58 shows the quartz cement built up as an initial thin zone followed by around sixteen separate growth generations (Nash et al., 2021) — direct evidence that cementation proceeded episodically through time. That doesn't by itself prove the boundary is spatially sharp, but it rules out "slow, steady, uniform thickening" as the model, and is consistent with a threshold-driven process.
3. Cementation chokes off its own further spread. As a patch of sand cements, its porosity and permeability collapse, diverting silica-bearing groundwater around the cemented zone rather than through it. Ongoing precipitation concentrates at the still-open margin instead of thickening a broad halo evenly. This permeability feedback — cementation progressively sealing off the flow that feeds it — is a generic feature of reactive transport in porous media, and it sharpens fronts rather than blurring them.
4. Most UK sarsens are groundwater silcretes, not pedogenic ones, and that distinction matters: groundwater silcretes form along specific subsurface flow paths and interfaces, favouring sharp contacts, consistent with UK sarsen's simple, structureless fabric and lack of pedogenic features such as geopetal or colloform structures. Pedogenic silcretes, which form within a soil profile through repeated wetting, drying and translocation, are generally understood to show more gradational or nodular boundaries tied to soil horizons — though I haven't found a source making that comparison explicitly for sarsen, so treat it as a reasonable extension of the general pedogenic-vs-groundwater silcrete literature rather than a confirmed point.
5. Timing: the Palaeocene–Eocene Thermal Maximum (PETM). Several independent strands of work link UK Palaeogene silicification — sarsen and the related Hertfordshire Puddingstone — to the PETM (c. 56–55.5 Ma), when elevated temperatures and weathering rates would have raised silica mobility and favoured rapid, localised precipitation (Worsley, 2019). This is a separate question from why the boundaries are sharp, but it supports treating UK sarsen formation as a comparatively brief, climatically distinctive episode rather than slow uniform diagenesis over millions of years.

Relevance to what follows. This isn't a fifth mechanism alongside the four below — it's the underlying reason several of them produce sharp-edged features rather than blurred ones:

• The sharp rim of a selective-dissolution void (mechanism 3) isn't created by the dissolution itself. It's inherited from the moment of cementation: the inclusion never took part in the silica cementation reaction, so the boundary between it and the surrounding cemented sand was already sharp the day the sarsen finished forming. Dissolution, much later, just empties out a void whose edge was sharp from the start.
• Root holes and burrow traces (mechanism 1) are preserved sharply for the same reason: the cementation front "freezes" whatever was already in the sand — including an open or sediment-filled tube — at the moment it reaches that point, rather than blurring it as growth proceeds.
• A sarsen boulder's own outer edge, where it meets the sand body it grew within, is the largest-scale expression of the same principle: a sharp boundary between sand that got drawn into the self-reinforcing cementation process, and sand just outside it that never did.

A note on terminology

"Gnamma" and "tafoni" are both sub-aerial weathering-pit terms — gnamma conventionally for granite, tafoni for sandstone — and both depend on processes that only operate at an exposed surface: rainwater ponding, lichen colonisation and the organic acids it produces, and freeze-thaw cycling. None of these operate on a stone that is buried. Applying either term to a depression that formed while a sarsen sat below ground is a category error, however similar the resulting cup-shape looks.

For the burial-context mechanism, the more accurate and lithology-neutral term is selective dissolution (sometimes "differential dissolution"): a softer or more soluble inclusion is preferentially removed from a chemically resistant host, leaving a negative cast. This is the same process documented in "omar" pits, where carbonate concretions dissolve out of Hudson Bay greywacke erratics. Reserve "gnamma" / "tafoni" for genuinely sub-aerial features; use "selective dissolution void" (or "primary void") for the burial-context equivalent.

Cup-shaped depressions in sarsens generally originate from four distinct mechanisms. The first predates the rock's induration entirely; the next two are post-lithification natural processes (one needing surface exposure, one needing burial); the last is human.

1. Primary Biogenic Structures (Root Holes & Worm Burrows)

These form within the original loose Palaeogene sand, before or during silicification — the cavity, or the trace of it, was already part of the sediment body when it hardened. This sets them apart from every other mechanism below, all of which act on the sarsen after it had already become rock.

Root holes (rhizoliths). A plant root grows down through the loose sand; when it later decays, it leaves a tubular void that becomes fossilised in place as cementation proceeds around it.

• Typically irregular and often tapering along their length, following the natural shape of a root rather than a true cylinder.
• May show smaller rootlets branching from a main channel.
• Walls are often knobbly/irregular rather than smoothly bored — a plausible source of the bumpy "mammillated" texture seen on some sarsen surfaces.
• Broadly vertical relative to the original ground surface at the time of growth, though exhumation and movement since can scramble the apparent orientation on a loose boulder.

Worm/invertebrate burrows (bioturbation). Burrowing animals active in the sand before it lithified left tunnels that are now preserved as traces — referred to formally as ichnofossils, with named genera such as Skolithos (simple vertical lined tubes) or Ophiomorpha (burrows with a distinctive knobbly, pelleted lining). Burrow ichnofossils are independently documented from the Sparnacian (basal Eocene) deposits of south-east England — the same general depositional package implicated in sarsen genesis — so they're a plausible, if not yet specifically confirmed for any individual sarsen, source for this kind of hollow.

• More uniform in diameter along their length than a root hole — a true tube rather than a taper.
• Orientation can be horizontal, inclined, or vertical depending on the producing organism's behaviour — less consistently vertical than root holes.
• May show a meniscate (stacked crescent) backfill structure in longitudinal section, or a distinct lining texture (e.g. Ophiomorpha's knobbly wall) — neither of which a root hole produces.

Distinguishing this category from mechanism 3 (below): there is no "missing inclusion" to account for. The cavity, or its sediment fill, was already present in that exact form before the rock hardened around it — it isn't evidence that something solid was once there and later dissolved away. A useful field check, where a fresh break or core is available: root holes and burrows typically continue as a recognisable tube into the rock at a fairly constant diameter, whereas a selective-dissolution void (mechanism 3) is usually a single, roughly isometric cavity the size and shape of one clast, not an elongated tube.

A root hole that happens to lie at or near the exposed surface can also act as the nucleating "seed" for a sub-aerial weathering pit (mechanism 2) — the two categories aren't mutually exclusive; one can be the starting point for the other.

2. Sub-Aerial Weathering Pits (Gnammas / Tafoni)

When sarsens are exposed on the surface, horizontal or gently sloping planes can develop weathering pits.

• Initiation: a structural "seed" — a localised pocket of incomplete silica cementation, a soft clay gall, or a root hole (see mechanism 1) — is generally required to start the pit.
• Mechanism: once a small depression is exposed, it acts as a micro-catchment for rainwater. Standing water, combined with humic/oxalic acids from endolithic lichens and freeze-thaw wedging, attacks the syntaxial quartz cement.
• Morphology: granular disintegration expands the pit into a bowl shape with a flared rim. The interior retains a rough, sandpaper-like texture, since dissolution removes the cement but leaves individual, un-sheared quartz grains palpable.

3. Sub-Surface Selective Dissolution (Primary Voids)

Sarsens that remained buried — in clay-with-flints, coombe rock, or other superficial deposits — are shielded from sub-aerial weathering but subject to constant sub-surface moisture.

• Mechanism: during Palaeogene silicification, migrating silica fluids frequently bypassed or encased non-siliceous inclusions — chalk clasts, clay galls, ironstone nodules, dense organic material. Over millennia, percolating groundwater dissolves or flushes out these softer inclusions.
• Morphology: because the surrounding silica matrix resists dissolution far more strongly than the inclusion did, the void does not expand into a bowl. It instead remains a comparatively faithful negative cast of the evacuated inclusion, with rim and interior geometry reflecting the original clast shape rather than the smooth, gravity-expanded geometry of mechanism 2.

4. Anthropogenic Modification (Cup Marks)

Human-made depressions, whether symbolic (rock art) or functional (grinding, polissoirs), can overlap in scale with natural pits — typically from a few centimetres up to around 20 cm in diameter.

• Mechanism: created through direct mechanical force — pecking, pounding, or grinding with another stone.
• Morphology: mechanical action fractures and shears quartz grains rather than dissolving the cement around them, producing a smoother, sometimes glazed or "bruised" interior with truncated grains — distinct from the loose, palpable grains of a dissolution feature.

Comparative summary

	Root hole / worm burrow (primary biogenic)	Gnamma / tafoni (sub-aerial)	Selective dissolution void (sub-surface)	Cup mark (anthropogenic)
Formed	Before/during lithification	After lithification, exposed	After lithification, buried	After lithification, human
Context	Within original sand body	Exposed surface	Buried	Usually exposed / portable stone
Shape	Tapering tube (root) or uniform tube (burrow)	Bowl, flared rim	Negative cast of inclusion	Hemispherical, regular
Interior texture	Knobbly (root) or lined/meniscate (burrow)	Rough, loose grains	Follows original inclusion surface	Smooth, glazed/bruised, sheared grains
Diagnostic check	Tube continues at constant width into the rock; no "missing clast" to explain	Lichen/weathering nearby	Burial history; matching inclusions elsewhere in matrix	Use-wear polish; fracture signatures under microscopy

Macroscopic description can suggest a category; confirming it generally needs field microscopy or thin-section work.