Calculating the bluestone route
Preseli to Stonehenge — sea or land?

Calculating the bluestone route

Set how costly water is next to walking on the flat. For every combination of settings the model has searched the terrain for the cheapest Preseli–Stonehenge route; the map shows the route it found. No route was drawn by hand. A sensitivity test — not a reconstruction.

overland on water ● onto or off water closed sea

This route

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Reading the result

  • Defaults (water = flat, 25 km loading charge, standard climb, boats in sight of land): all inland — Llandovery, Brecon, Monmouth, round the Severn head. The A40 line. Sea only wins once water is roughly a fifth cheaper than walking.
  • Drop the loading charge and sea and land tip at about equal cost per km (about 1.03 in finer runs). Both routes are ~239 km; the hills barely decide it. Just above that, the “land” path still hops the south Wales estuaries.
  • No loading charge, water 2–5× dearer: Tywi, across the coalfield to Newport, then a short Severn ferry at Beachley–Aust.
  • Staying in sight of land barely changes the answer. A hard 2 km limit does.
  • The two outcrops match in 466 of 468 settings. The odd cases are cheap water: Rhos-y-felin north round St David’s Head; Carn Goedog south to Carmarthen Bay.
  • Land’s End only when water is about a tenth of flat ground or cheaper.

How the cost works

overland d + c × climb + 0.65c × descent on water m × d loading + T each time onto or off water

d is distance in metres, c the climb penalty, m the water multiplier and T the loading charge. OS Terrain 50 at 200 m; sea = cells mostly below 0 m OD. The settings come first: each of the 936 combinations (both outcrops) was run in advance, and Dijkstra’s algorithm found the cheapest path across the terrain for it. The page only looks up the answer.

What the model leaves out

  • Tides, currents, weather, seasons and the risk of losing a stone.
  • How the stones were carried: no sledges, boats, rafts or crew sizes.
  • Vegetation, woodland and wetland. Terrain 50 is today’s surface.
  • Soft or flooded valley floors, and the cost of crossing rivers. River valleys still attract routes because they are flat.
  • Mid-Holocene sea level. It was already within a few metres of present, and on these steep coasts that barely moves the shoreline at this grain.
  • And it does not claim the cheapest route is the one used.

Protocol, amendments, scripts and every run: bluestone-sea-dial repository. Background: natural route analysis and the natural corridor for bluestones.