Tuesday, 6 October 2026

Sea or land? Calculating the bluestone route

This post goes with the short film Sea or land? and with a model anyone can run in a browser:

Calculating the bluestone route. (click to go to the model)

In short

Did the bluestones travel from the Preseli hills to Stonehenge by sea or over land? The model weighs the hard parts of the journey: distance, hills, time on the water, and lifting stones of several tonnes into and out of boats. Each can be turned up or down. For every setting, the model searches the ground for the easiest route; no route is drawn in advance and then fitted to the settings.

  • If boats are easy, the sea wins: down to Carmarthen Bay, then up the Bristol Channel to Avonmouth.
  • If loading and landing are hard work, the land wins, along the line of today's A40 through Llandovery, Brecon and Monmouth. On the most reasonable settings, this is the answer.
  • Many land routes are almost as easy, and all of them cross the Severn at one place: Newnham, below Gloucester.
  • The two Preseli outcrops, one high on the ridge and one lower down, give the same answer almost every time.
  • Keeping boats in sight of land, as Atkinson assumed, makes no difference.

None of this says where the stones actually went. Tides, weather, boats, crews and much else are left out. The model puts numbers on the argument, so that anyone can see what each assumption implies.

The question

Bluestone dolerites and rhyolites at Stonehenge have been matched to outcrops on the north side of Mynydd Preseli, among them Carn Goedog (Bevins, Ixer & Pearce 2014) and Craig Rhos-y-felin (Parker Pearson et al. 2015). Carn Goedog is 219 km from Stonehenge in a straight line. Proposed routes fall into two groups: by sea along the south Wales coast and up the Bristol Channel, as Atkinson proposed, or overland through south Wales and across the Severn. The overland case has been made here before, in The natural corridor for bluestones and Natural route analysis.

A least-cost model can compare the two only if water has a price. Make the sea free and the answer is the sea; make it a wall and the answer is the land. Published corridor work does the second: in Lewis (2024) the sea lies outside the cost surface. The question here is therefore: how cheap must water travel be, relative to hauling over land, before the cheapest route takes to the sea?

The model

The ground is OS Terrain 50, averaged onto a 200 m grid from Pembrokeshire to Hampshire and south to Land's End. Each step to a neighbouring cell is costed in metres of flat-ground hauling:

  • Overland: distance, plus 4 × metres climbed, plus 2.6 × metres descended.
  • On water: distance × m, the water cost. At m = 1 a kilometre on water costs the same as a kilometre on the flat; at 0.5, half as much.
  • Loading: an optional fixed charge T each time the load goes onto or off the water.

The climb weight of 4 corresponds to hauling against a friction coefficient of 0.25: if the force required scales as μ + gradient, the cost per metre scales as 1 + gradient/μ. Sea is any cell where most Terrain 50 posts lie below 0 m OD or where there are no posts offshore. Dijkstra's algorithm finds the cheapest route. A route counts as seaborne when at least 25% of it is on water, and as overland when no more than 10% is.

The order of work matters. First the settings were fixed: two outcrops, thirteen water costs, four loading charges, three climb weights and three rules for keeping to the coast, 936 combinations in all (468 from each outcrop). Then, for each combination, the model searched the whole cost surface and kept the cheapest route it found. No route was chosen or drawn beforehand: each one is simply the output for its settings, and different settings often return the same route. Because the full set is too slow to run in a browser, it was run in advance, and the model page looks up the stored result for whatever settings are selected. Scripts and full results are in the project repository.

Several other settings were tested and left off the page because they barely move the result. A climb weight anywhere from 1 to 16 puts the sea–land switch at m = 1.01–1.12. Closing slopes steeper than 1 in 10, or treating the main rivers as barriers or as waterways, leaves it at 1.03. Grids of 400, 200 and 100 m give 1.027, 1.030 and 1.037. Allowing 16 moves per cell instead of 8 raises it to 1.05–1.08.

The cheapest route: the A40


The cheapest route with water at 1 × flat ground and a charge of 25 km of hauling for each loading and each landing: Llandovery, Brecon, Monmouth, round the head of the Severn estuary. Model run, 200 m grid.

The model page opens with water at the same cost as the flat, a charge of 25 km of hauling for each loading and each landing, the standard climb weight, and boats kept in sight of land. On these settings the cheapest route never touches water. It runs from Carn Goedog up the Tywi to Llandovery, on to Brecon and Monmouth, and round the head of the Severn estuary: 256.5 km, the line of the A40. For the sea to win, water must be about 20% cheaper per kilometre than the flat (the switch lies between m = 0.79 and 0.81).

The loading charge decides this. Lifting a stone of several tonnes onto a boat and off again is not free, and without a charge the model lets a load hop on and off the water at no cost. With the charge, the land route avoids even a short ferry across the Severn. The right size of charge is open. Of the 117 settings per outcrop at each charge, the A40 is the answer in 9 with no charge, 24 at 10 km, 66 at 25 km and 80 at 50 km.

One crossing


Ground lying on some route within 1% (dark red), 2% (orange) or 5% (yellow) of the cheapest, from Carn Goedog, on the model page's default settings.

A single cheapest line overstates what the ground decides. A better measure is the band of near-cheapest routes: every cell on some route costing no more than 1%, 2% or 5% above the cheapest. For a 256 km route, 1% is about 2.5 km of hauling.

On the default settings the 1% band is 15–20 km wide from Preseli to Brecon and splits round the hills into a Teifi side and a Tywi side. The ground does not pick one valley. It does pick one crossing: every near-cheapest land route crosses the Severn at Newnham, about 12 km below Gloucester, where on a 200 m grid the estuary has narrowed to river width. The model charges nothing for crossing a river, so this marks where the estuary stops being sea in the terrain data, not a known ford or ferry. The historic lowest bridge was at Gloucester. East of the Severn the band widens again towards Stonehenge.


Near-cheapest band with water at 2 × flat ground and no loading charge. Bands as above.

With water at twice the cost of the flat and no loading charge, the band runs straighter and funnels instead to the short crossing at Beachley–Aust. Either way the model identifies a crossing place, not a single line through Wales.

The sea's case


Share of the cheapest route on water as the water cost changes (Carn Goedog, 200 m grid, no loading charge). The route flips at m = 1.03.

Without a loading charge the balance is almost exactly even. The route switches from sea to land at m = 1.03 for both outcrops: the sea wins whenever a kilometre on water costs no more than about a kilometre on the flat.

The reason is geography. The Bristol Channel lies almost on the straight line from Preseli to Stonehenge, and at the switch the last sea route and the first land route are both 238.9 km long. Climbing adds about 2% to the sea route and about 9% to an all-land route, so the hills of south Wales hardly count. What decides the outcome is the cost of water relative to the flat, and a terrain model cannot supply that.


Water at half the cost of the flat, no loading charge: the route boards in Carmarthen Bay and lands at Avonmouth. Model run, 200 m grid.

With water at half the cost of the flat and no loading charge, the route from Carn Goedog walks 15–20 km to Carmarthen Bay, boards near Pendine, sails up the Bristol Channel and lands at Avonmouth: 259 km, 61% on water.


Water at twice the cost of the flat, no loading charge: the route walks and crosses the Severn at Beachley–Aust. Model run, 200 m grid.

At twice the cost it walks: up the Tywi, across the coalfield valleys towards Newport, over the shortest Severn crossing (3.5 km, Beachley to Aust) and on through Bath: 238 km, 1.5% on water. That route holds up to water at ten times the cost of the flat. The A40 line takes over only when loading is charged or water is very dear.

Two outcrops

Carn Goedog sits on the ridge at about 290 m OD, 9 km from the sea at Newport Bay. Craig Rhos-y-felin lies in the valley below at about 80 m, 6 km from the same bay. Across 468 combinations of settings the two agree on sea or land in all but two, and the switch is the same. They differ when water is cheap. At a fifth of the cost of the flat, the route from Craig Rhos-y-felin goes north to Newport Bay and round St David's Head (345 km), while the route from Carn Goedog walks south to Carmarthen Bay (260 km). Over the whole set, Craig Rhos-y-felin goes round the north 100 times and Carn Goedog 80.

Keeping to the coast

Atkinson's sea route assumed boats kept close inshore, in sight of land. The model tests this by closing sea cells. Under the in-sight rule a sea cell stays open only if land shows above the horizon from a boat with an eye height of 2 m, that is, within 3.57 × (√2 + √H) km of land H metres high. A stricter rule closes all sea more than 2 km from land.

Staying in sight of land changes nothing: from every stretch of sea these routes use, land is above the horizon, even round Land's End. Limits of 20 and 10 km also leave the switch unchanged, and 5 km moves it slightly, only with a loading charge (to 0.77–0.78). The 2 km limit lowers it to 0.94–0.97 without a charge and 0.67–0.68 with one, because a voyage hugging the shore is longer. In the Bristol Channel land is never far away, so keeping to the coast hardly affects the choice.

Round Land's End?

With the whole coast open, a route north to Newport Bay and round St David's Head and Land's End wins only when water costs about a tenth of the flat or less. At a twentieth it sails 685 km and lands at Southampton Water. Within 2 km of land it is never the cheapest route.

What the model leaves out

  • Tides, currents, weather, seasons, and the risk of losing a stone at sea or on land.
  • How the stones were moved: no boats, rafts, sledges, rollers or crew sizes are modelled.
  • Vegetation, woodland and wetland. OS Terrain 50 is today's land surface.
  • Soft or flooded valley floors, and the cost of crossing rivers. Valleys still attract routes because they are flat: 15.6% of the default land route lies in the main river corridors, which cover 2.2% of the land in the frame.
  • Mid-Holocene sea level. By then relative sea level around the Bristol Channel was within a few metres of today's (Shennan, Bradley & Edwards 2018), not tens of metres lower as in the early Holocene. Most of these coasts are steep, so a few metres barely moves the shoreline on a 200 m grid. The Severn levels near Avonmouth and Weston-super-Mare are the exception, where the tidal edge may have lain some way inland. That changes where a stone could come ashore, not whether water beats land.
  • Any claim that the cheapest route is the one used.

What follows

Once loading and landing cost something, the cheapest way from Preseli to Stonehenge is overland on the A40 line, through a broad band across Wales and a single crossing of the Severn at Newnham. Without that cost the ground cannot choose: sea and land routes are almost the same length, and the break-even sits at water costing about the same as the flat. Whether Neolithic water transport of a multi-tonne stone, loading and landing included, was cheaper than that is a question for experiment and for the archaeology of boats and landing places, not for elevation data. The model makes the price explicit and lets anyone change it: try the model.

Notes on the film

Every route in the film is a model run on the 200 m grid with 8 moves per cell, drawn as the model gives it, over an OS Terrain 50 hillshade. The near-cheapest band uses the model page's default settings. The switch chart shows the share of each cheapest route on water for 22 water costs. In the film, "boat 2× easier" means m = 0.5, "same effort" means m = 1, and "if loading boats is hard" means a charge of 25 km of hauling for each loading and each landing. The music is original, generated for the film.

Sources

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Monday, 5 October 2026

Aurochs and Aurochsen - a short cartoon reminder

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Stones followed people

The previous post (Metal Followed People?) found that Britain's Beaker-period incomers came from the Rhine while its first copper came up the Atlantic. A new preprint (McColl, Schulz Paulsson et al. 2026) asks the same question two thousand years earlier, of megaliths. It models the ancestry of 2,963 ancient Europeans, including 46 new genomes from megalithic and related burials in Portugal and Sardinia, against the radiocarbon chronology for megalith emergence. The test is simple: if megaliths spread with migrants, their appearance in a region should coincide with an influx of ancestry from a region already building them.

Click to enlarge

Britain: the monuments came with the people

Northern France, where the earliest megaliths appear around 4750 BC, carried a distinctive admixed ancestry, mostly of Western Mediterranean (Cardial) origin with a minor Central European (LBK) component. Every Neolithic individual sampled from Britain and Ireland models as that northern French ancestry plus a little local hunter-gatherer input, with almost none of the Iberian Cardial signal. The migration dates to about 4150–4050 BC and megaliths follow by about 3950 BC. Of the regions tested, only Britain and Ireland meet the migration expectation. The precise source remains open: no genomes survive from Brittany between about 4950 and 3850 BC. The earthen Passy monuments of the Paris Basin, built by people of the same ancestry from about 4750 BC, make the British earthen long barrow a plausible inheritance rather than a stoneless substitute.

Elsewhere, the monuments travelled without them

Western Iberia acquired passage graves indistinguishable from Britain's at the same date, yet the new Portuguese megalith burials are of local Iberian ancestry, with no rise in the northern French component in either phase of expansion. In Scandinavia, farmers carrying northern French–derived ancestry arrived around 4100 BC, but dolmens appear only around 3650 BC and passage graves around 3400 BC, with no fresh influx to account for the timing. The authors argue that building knowledge cannot have lain dormant for centuries and propose transmission by specialist builders moving through exchange networks. That argument overlooks the earthen long barrows built from early in the Danish and north German Funnel Beaker sequence. A local translation of an existing monument form into stone fits the delay equally well.

Jadeite as the network

Alpine jadeitite plays the part that copper plays in the Beaker story. From about 4750 BC it reached Brittany, which became a redistribution centre feeding Iberia. Quarrying in the western Alps had largely ceased by about 4350 BC, yet jadeitite axes reached the British Isles and southern Scandinavia from about 4150 BC and remained in circulation until about 3550 BC. Megaliths emerged only within the reach of these networks, and only among populations of Cardial-derived ancestry, never among persisting hunter-gatherers or LBK descendants.

The inversion

Britain's two transitions share a structure. Around 4000 BC people and monuments arrived from northern France while prestige stone came through a separate network. Around 2450 BC people arrived from the Rhine while copper came up the Atlantic. In both, Iberia sent materials north (variscite and fibrolite then, copper later) without sending a detectable population. The cross-tabulation proposed for Beaker metal, grave goods against the ancestry of the buried, has now been done for megaliths: in Iberia the monument tradition crossed the ancestry divide.

Caveats

The paper is a preprint. Its northern French reference population is two Cerny individuals from the region where megaliths began, which builds part of the association into the design. The survival model linking ancestry to megalith emergence rests on six regional events. A handful of incoming specialists would be invisible to genome-wide ancestry modelling, so "no migration" in Iberia means none detectable.

Sources
McColl H, Schulz Paulsson B, Shennan S, et al. 2026. Migrations facilitated the cultural diffusion of megaliths in Neolithic Europe. bioRxiv preprint, posted 1 October 2026. doi:10.64898/2026.09.24.753699
Merkel SW, Arnoldussen S, Theunissen L, et al. 2026. Tracking Europe-wide copper and bronze flow in the Dutch Late Neolithic to the Early Iron Age. PLOS One 21(9): e0348751. doi:10.1371/journal.pone.0348751
Olalde I, Brace S, Allentoft ME, et al. 2018. The Beaker phenomenon and the genomic transformation of northwest Europe. Nature 555: 190–196. doi:10.1038/nature25738

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Sunday, 4 October 2026

Where would the aurochs drink? Herds, springs and the Stonehenge car-park posts

What we know

Blick Mead, east of Stonehenge, is a chalk spring where Mesolithic people hunted aurochsen and came back for a very long time (see Buried Landscapes of the Avon). On the ridge to the west are the big Mesolithic posts found under the old car park. Five radiocarbon dates on their pine charcoal span about 8820–6590 cal BC (Allen & Bayliss 1995, in Bronk Ramsey & Bayliss). Nearby runs the palisade ditch, which is poorly dated. It is put in the Late Neolithic on archaeological grounds, perhaps 2900–2400 BC, and the only date is from an Iron Age burial cut into it, 780–410 cal BC (Lawson 1997; Bronk Ramsey & Bayliss). The posts and the palisade are something like 4,000 to 6,000 years apart. I have suggested that the posts held machans, raised hunting platforms like those E. P. Stebbing drew in India. I have also wondered whether a palisade with gaps worked like a bull-run fence. Both ideas need herds to pass by.

So where would herds actually walk to water? On the chalk, the answer depends on winter. The climate of the period was much like today's, and the Avon gets frosty in a cold spell but doesn't freeze solid (see Britain, 5000–2000 BC: no ice, no catastrophe, a familiar climate). A spring, meanwhile, stays open. So the model below gives open springs an advantage in frost: a herd will walk 0.5, 1 or 2 km further to reach one.

How cattle walk

The rules come from studies of domestic cattle, not aurochsen, but they are the best measurements there are.

  • Climbing is costly. On a treadmill, cattle used about 26 J per kg for each metre climbed, against 2 J per kg per metre on the flat (Ribeiro et al. 1977). So a metre of climb costs as much as about 13 metres on the level.
  • Going down costs too. Steers walking down a 6% grade used about as much energy as walking up it (Di Marco & Aello 1998).
  • Steep ground is avoided. Cattle start avoiding slopes of 20% (Ganskopp & Vavra 1987). Land over 60% is treated as ungrazable (Holechek 1988), and the model treats it as impassable.
  • Distance from water matters. The rangeland rule of thumb is full use up to 1.6 km from water and half use from 1.6 to 3.2 km (Holechek 1988). GPS-collared cattle often go further than that (Millward et al. 2020).
  • Trails find the easy line. Real cattle trails from water were 11% shorter than GIS least-effort paths. They ran on slopes of 5.2% across pastures that averaged 13.5% (Ganskopp et al. 2000).

The model puts these rules on Environment Agency LiDAR at 5 m resolution. It starts 2,145 herds on the high ground, one every 250 m wherever the ground is at least 25 m above a river. Each herd takes the cheapest route to water. The water is the Avon, the Till, Blick Mead, and five springs marked on the 1880s OS six-inch maps: four at Winterbourne Stoke and one at Durrington. Where many routes share a line, that is a busy track.

Results

The posts are not on a track. In every run, no busy route (one used by 25 or more herds) reaches them. In the hardest frost the closest busy route comes within 615–730 m of the posts when Blick Mead is the only spring (Figure 1). With the OS springs added (Figure 2), the closest is about 1.3 km away (1.28–1.39 km).

They sit on the edge of Blick Mead's winter catchment. With no advantage for springs, the ground around the posts drains to the lower Avon. Give the springs the smallest advantage tested, 0.5 km, and the same ground switches to Blick Mead. It stays with Blick Mead at every frost setting, with or without the other springs. The posts look more like a watch point over herds drifting towards the spring in winter than an ambush on a path.


Figure 1. Cattle walking rules in a hard frost (open springs worth a 2 km detour; Blick Mead the only spring). Purple marks the ground whose herds drink at Blick Mead: 463 of 2,145 herds, from 37.7 km². The busy routes converge on the spring but pass the posts at a distance.

Blick Mead is not the only draw. In the hardest frost tested, with all six springs, Blick Mead takes 328 herds from 27.9 km², the most for any single spring. The Winterbourne Stoke springs take 268, 261, 52 and 34, so 615 between them, and Durrington takes 182. In milder frost a Winterbourne Stoke spring beats Blick Mead: 142 herds to 90 at 0.5 km, and 185 to 128 at 1 km. Blick Mead is the busiest spring only at the hardest setting. Even so, the posts stay in its catchment.


Figure 2. The same hard frost with the five springs from the 1880s OS six-inch maps added (green dots; red marks herds going to those springs). The Winterbourne Stoke group takes much of the Till side, but the posts remain in Blick Mead's catchment (purple).

The palisade is not a funnel. Few routes come near it. Making it a solid barrier changes almost nothing: in any run, at most one of the 2,145 routes changes its water. If it steered animals, it was in a drive, which this model cannot test.

Limits

This is a simple exploratory model, not proof. There are no GPS tracks to test it against. The rules come from cattle, and bison, the nearest wild comparison, avoid slopes more strongly and are less tied to water (Allred et al. 2011). The rivers are today's, from OpenStreetMap. The springs are Victorian map labels placed to within ±40–60 m. The posts and the palisade are thousands of years apart, so the model puts them on one landscape only for comparison. Some settings are my own choices, such as the steepness penalty reaching ×10 at a 60% slope.

GPS tracks of cattle grazing chalk downland for conservation would be the real test.

References

Data: EA LiDAR Composite DTM 1 m © Environment Agency (OGL v3). Palisade ditch (HE_UID 219850) and Avenue from Historic England Aerial Investigation & Mapping, © Historic England (OGL v3). Rivers and the Blick Mead point © OpenStreetMap contributors (ODbL). Springs from the OS six-inch maps, reproduced with the permission of the National Library of Scotland, and GB1900 (CC BY-SA). Car-park post positions after Vatcher & Vatcher 1973 and Allen 1995 (HE Monument 219856).

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Metal Followed People?

Between about 2450 and 2000 BC, roughly 90 per cent of Britain's ancestry was replaced by incoming groups associated with Beaker material culture, whose closest genetic relatives lived in the Lower Rhine region (Olalde et al. 2018). A new lead isotope and trace-element study of 201 copper-alloy objects from the Netherlands and Flanders (Merkel et al. 2026) samples metalwork from that source region. It therefore gives an indirect view of what was crossing the North Sea during the turnover.

Reconstruction of metalwork flows for the different phases (a–d). The routes suggested include the use of the major river systems (Rhine, Weser, Elbe, Oder and their tributaries) (maps: S. W. Merkel). From Merkel et al. 2026, Fig 19, CC BY 4.0. Base map © Esri. Sources: Esri, TomTom, Garmin, USGS, FAO, NOAA. Map image is the intellectual property of Esri and is used herein under license. Copyright © 2026 Esri and its licensors. All rights reserved. Reprinted under CC BY license with permission from Esri (static maps terms).

Copper before the incomers

Copper circulated on the near continent long before it appears in Britain. The earliest Dutch object analysed, a flat axe from Glanerbrug typologically dated to 4000–2500 BC, is attributed to Serbian copper. A double axe from Escharen (2500–2000 BC) is attributed to Slovakian ore. Neolithic Britain was not isolated: Alpine jadeitite axes were crossing the Channel in the fourth millennium. Yet no securely dated copper is known in Britain before Beaker-associated burials. On present evidence, copper did not reach the island's Neolithic population as a trade good before the incomers arrived.

People from the Rhine, metal from the Atlantic

The Dutch data show two copper streams converging in the late third millennium. The Wageningen hoard, tentatively dated to 2200–2000 BC, is mostly Únětice-type fahlore copper from Central Europe. Its halberd and dagger, however, are Iberian arsenical bronze, and the halberd blade is fixed with Central European rivets. A further seven arsenic-rich axes and daggers of 2000–1800 BC, several of British, Irish or Armorican type, are tentatively assigned to Argaric copper from south-east Iberia. That comparison does not consider Ross Island in Co. Kerry, the principal Beaker-period copper source in Ireland.

Britain shows the same decoupling. Its migrants came mainly from Rhine-delta populations, but its earliest copper arrived by Atlantic routes. Most came from Ross Island, and a few highly radiogenic objects from Britain's earliest metal-using phase probably derive from the Asturian mines of northern Spain.

Trade with whom?

Lead isotopes trace metal, not people. They cannot show whether copper in Britain during the turnover circulated only among incomers or also reached surviving Neolithic communities. The test lies in combining grave context with ancient DNA. Copper in the burials of individuals of predominantly Neolithic ancestry would indicate exchange across the population divide. No such cross-tabulation has been published. The nearest case is burial 25004 in the Boscombe Bowmen grave at Boscombe Down (24th century BC), which has the highest British Neolithic ancestry of any Chalcolithic–Early Bronze Age individual sampled; one parent was probably wholly of Neolithic descent (Booth et al. 2021). The grave held seven or eight Beakers, flint arrowheads, a boar's tusk and a bone toggle, but no metal. Beaker material culture therefore crossed the ancestry divide within the first generations, but whether metal did as well remains untested.

The return flow

British metal reached the continent only after the post-turnover population had developed its own mines. British copper becomes prominent in the Netherlands after 1800 BC, when Great Orme copper dominates the supply. Examples include the British-type Acton Park palstaves from the Voorhout hoard and a Wessex II Camerton-Snowshill dagger, probably of Great Orme metal, from a barrow at Annertol. The export trade belongs to the incomers' descendants, centuries after the replacement.

Sources
Merkel SW, Arnoldussen S, Theunissen L, et al. 2026. Tracking Europe-wide copper and bronze flow in the Dutch Late Neolithic to the Early Iron Age. PLOS One 21(9): e0348751. doi:10.1371/journal.pone.0348751
Booth TJ, et al. 2021. Tales from the supplementary information: ancestry change in Chalcolithic–Early Bronze Age Britain was gradual with varied kinship organization. Cambridge Archaeological Journal 31(3): 379–400. doi:10.1017/S0959774321000019
Olalde I, Brace S, Allentoft ME, et al. 2018. The Beaker phenomenon and the genomic transformation of northwest Europe. Nature 555: 190–196. doi:10.1038/nature25738

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Saturday, 3 October 2026

Stonehenge in Six Questions

Kipling kept "six honest serving-men": What and Why and When, and How and Where and Who. Put to Stonehenge, five of them have answers backed by evidence. This page is the companion to the short film Stonehenge in six questions, with the sources behind each card.

Where

On Salisbury Plain in Wiltshire, about two miles (3.3 km) west of Amesbury and eight miles north of Salisbury, in a landscape dense with Neolithic and Bronze Age monuments, above all round barrows. Durrington Walls, about two miles to the north-east, is a far larger henge; houses excavated there may have sheltered the people who built Stonehenge (Parker Pearson et al. 2007).


Still from the Stonehenge Landscape Flyover.
(Click Photos to embiggen)

What

A circular bank and ditch, the earliest part of the monument. Inside it, a ring of sarsen uprights capped by a continuous circle of lintels; within that, a horseshoe of five trilithons; and among them the smaller bluestones, set as a circle and an inner horseshoe. Stonehenge gave its name to the class of monuments called henges, yet fails the definition: its main bank lies inside its ditch, not outside (see Is Stonehenge a henge?).


Plan from the Stonehenge 3D model, bluestones in blue. T. Daw, after Rees 1989.

When

The bank and ditch were built about 3000 BC. The sarsens and bluestones were set up in the centre about 2500 BC; the bluestones were rearranged two or three centuries later; and the Y and Z holes around the circle are dated between 1800 and 1500 BC (English Heritage; Darvill et al. 2012 set out five stages).

How

Fifty of the fifty-two surviving sarsens share a geochemical signature with West Woods, about 25 km to the north; typical uprights weigh around 20 tonnes (Nash et al. 2020). The bluestones come from the Preseli Hills in west Wales, some 220 km away in a straight line. The Altar Stone is a sandstone from much further north: Clarke et al. (2024) point to the Orcadian Basin of north-east Scotland. On the evidence so far that is a search area rather than a source outcrop (see The Altar Stone Zircons, One Sample at a Time). How the stones were moved is still debated; the distances here are straight lines, not routes.


Sarsens from West Woods (25 km), bluestones from the Preseli Hills (220 km). Lines are not routes.

Who

The builders were farmers whose ancestors had crossed from the continent around 4000 BC; ancient genomes show that Britain's Neolithic transition "was mediated by incoming continental farmers, with little gene flow from local hunter-gatherers" (Brace et al. 2019). About 64 cremations have been found at Stonehenge, and perhaps as many as 150 people were originally buried there (English Heritage). Isotopes in pig bones from Durrington Walls suggest animals were brought from across Britain for feasts, though the authors note that "it is not possible to define origins with confidence" (Madgwick et al. 2019). From about 2450 BC newcomers associated with Beaker pottery arrived, and within a few centuries about 90 per cent of Britain's gene pool had been replaced (Olalde et al. 2018).


Reconstructed Neolithic house, Stonehenge visitor centre. Alan Hunt, geograph.org.uk/p/4520192, CC BY-SA 2.0.

Why

Unknown. The one fixed clue is the axis: from the centre, the midsummer sun rises just to the left of the Heel Stone, and the midwinter sun would originally have set between the uprights of the tallest trilithon; the final straight stretch of the Avenue follows the same north-east to south-west line (English Heritage). Proposed purposes include a temple to the sun, a monument to the ancestors (Parker Pearson & Ramilisonina 1998) and a place of healing (Darvill 2006). The evidence does not yet choose between them.

Notes on the film

  • Dates are rounded from radiocarbon ranges.
  • Bluestone positions on the plan are taken from the stone-number points on the Rees 1989 sheet (via the georeferenced plan); they mark positions, not shapes.
  • The maps use the NOAA GLOBE land mask. Lines join a source to Stonehenge; they are not routes.
  • Music: original.

Sources

  • Brace, S. et al. 2019. Ancient genomes indicate population replacement in Early Neolithic Britain. Nature Ecology & Evolution 3: 765–771.
  • Clarke, A. J. I. et al. 2024. A Scottish provenance for the Altar Stone of Stonehenge. Nature 632: 570–575.
  • Cleal, R. M. J., Walker, K. E. & Montague, R. 1995. Stonehenge in its Landscape: Twentieth-Century Excavations. London: English Heritage.
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Friday, 2 October 2026

The Dorset Cursus and the midwinter sun: a LiDAR test

This post is the companion to the short film Ten kilometres for one sunset?, which tests a fifty-year-old astronomical claim about the Dorset Cursus against modern LiDAR. The method, numbers and caveats follow.

Fly the cursus yourself: Dorset Cursus 3D LiDAR flyover.

The monument

The Dorset Cursus is a pair of parallel banks with outer ditches running for just over 6¼ miles, nearly 10 km, across Cranborne Chase, from Thickthorn Down in the south-west to Martin Down in the north-east (RCHME 1975; Dorset HER MDO5608). It was built in two parts. The first, the Gussage cursus, ran from Thickthorn to a terminal on Bottlebush Down; the second, the Pentridge cursus, continued it on a slightly different alignment to Martin Down. Measured along the published points, the two are 5.65 km and 4.33 km long. Radiocarbon dates span the second half of the fourth millennium BC (Bowden et al. 1983; Barrett, Bradley & Green 1991). For most of its length it survives only as a cropmark or soilmark; the ends at Thickthorn and Martin Down remain as earthworks.

Long barrows lie beside it, inside it and in its banks. On Gussage Hill one lies across the interior, at right angles to the banks: a mound 155 ft long, 65 ft across and up to 10 ft high (RCHME 1975; Dorset HER MDO5615).


The Dorset Cursus on EA 1 m LiDAR, from the south-east. Height ×1.5.

The claim

Penny and Wood (1973) proposed that an observer at the Bottlebush terminal of the Gussage cursus would see the midwinter sun set behind the Gussage Hill long barrow. Barrett, Bradley and Green (1991) accepted this as the most plausible of Penny and Wood's alignments. It has since become the standard astronomical reading of the monument.

The test

The Environment Agency's LIDAR Composite DTM at 1 m (2022) covers the whole cursus. The observer is placed 30 m inside the Bottlebush terminal (RCHME SU 01581566) on the line of the Gussage cursus, at E401558 N115639, with the eye 1.6 m above the ground. The barrow mound is located directly in the DTM: its summit stands 3.1 m above its surroundings, and the part higher than 0.6 m is 43 m long on a bearing of 143°, matching the RCHME description.

From the observer the barrow is 2.89 km away on a bearing of 231.2°; the mound spans 230.8° to 231.7°. Sight lines are traced across the DTM out to 8 km with the curvature of the earth and terrestrial refraction (coefficient 0.13) included.


Section along the sight line from the Bottlebush terminal. The barrow is the skyline; the ground beyond stays below the line. Height exaggerated as marked.

The barrow is the skyline. On bearings from 224° to 238° the horizon is the Gussage Hill ridge, 2.83 to 2.95 km away, and the mound lifts it from about 0.24° to 0.29° above level. Nothing further away shows above it.

The sun. In 3300 BC the obliquity of the ecliptic was 24.05° (Laskar 1986), so the midwinter sun's declination was −24.05°. At latitude 50.93° N, with standard refraction at the horizon's altitude, the last gleam of the sun, its upper limb touching the skyline, goes down on a bearing of 230.60°. Moving the date to 3500 BC or 3000 BC shifts this by less than 0.05°.


The skyline from the middle of the Bottlebush terminal at true angular scale, with the midwinter sun of 3300 BC at three moments of its setting. The last gleam goes down just south of the mound.

From the middle of the terminal it just misses. The last gleam goes down 0.20° south of the mound's south-west end, about 10 m at the distance of the barrow. The sun's disc is 0.53° across, so the miss is less than half its width.

From the north-west half it works. Moving across the cursus changes the barrow's bearing by about 0.02° for every metre. From anywhere between 10 m and 55 m north-west of the axis, which is the north-west half of the terminal, the last of the midwinter sun goes down behind the mound.


The Bottlebush terminal on LiDAR local relief. Gold: the last gleam goes down behind the barrow. Blue: it goes down just beside it. Banks as traced on the flyover.

What could move it

  • Refraction. Near the horizon in winter it varies from night to night; a change of 30% moves the last gleam by about 0.27°, more than the miss from the middle of the terminal.
  • The terminal. The Gussage terminal bank on Bottlebush is ploughed. Its position comes from an eight-figure RCHME grid reference, and the axis there from the line between published points, not from surviving banks. Either can shift the zone by several metres.
  • The barrow. The DTM records the mound as it is now, ploughed and eroded. A taller mound would widen the zone, not narrow it.
  • Trees. Neolithic woodland on Gussage Hill, if any, is unknown and not modelled.

Conclusion

The 1973 claim survives a test its authors could not run. The Gussage Hill long barrow is the skyline from the Bottlebush terminal, and the midwinter sun of the late fourth millennium BC sets behind it when seen from the north-west half of the terminal, and just beside it from the south-east half. Whether the builders intended this cannot be settled from the ground.

How the line is drawn. No complete survey plan of the cursus is published. The banks on the flyover run between the published positions of the terminals and of points along the line (Historic England 1002785; RCHME long barrow 14, SU 011152, SU 01581566, SU 025169, SU 04051920), moved onto the ridges where the LiDAR still shows them, and left on the straight line between points across ploughed gaps. The code for the test is in the film's folder on GitHub (align.py, sens.py, diagrams.py).

References

  • Atkinson, R. J. C. 1955. The Dorset Cursus. Antiquity 29, 4–9.
  • Barrett, J., Bradley, R. & Green, M. 1991. Landscape, Monuments and Society: the prehistory of Cranborne Chase. Cambridge University Press.
  • Bowden, M., Bradley, R., Gaffney, V. & Mepham, L. 1983. The date of the Dorset Cursus. Proceedings of the Prehistoric Society 49, 376–379. doi:10.1017/S0079497X00008069
  • Dorset Historic Environment Record: MDO5608 The Dorset Cursus; MDO5615 Long barrow on Gussage Hill.
  • Historic England list entry 1002785, Western end of Dorset Cursus.
  • Laskar, J. 1986. Secular terms of classical planetary theories using the results of general theory. Astronomy and Astrophysics 157, 59–70.
  • Loveday, R. 2019. Neolithic cursus monuments. Prehistoric Society, Neolithic factsheet 8.
  • Penny, A. & Wood, J. E. 1973. The Dorset Cursus complex: a Neolithic astronomical observatory? Archaeological Journal 130, 44–76.
  • RCHME 1975. An Inventory of Historical Monuments in the County of Dorset, Volume V: East Dorset. HMSO.

LiDAR: Environment Agency LIDAR Composite DTM 1 m, 2022. © Environment Agency copyright and/or database right 2022. Open Government Licence v3.0. Flyover, diagrams and analysis: T. Daw. Music in the film: original.

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