BGS DigMapGB-625 bedrock (OGL). DEM: AWS Terrarium z12. Internal analysis — not for publish.
| n | % chalk | elev median (m) | prominence median (m) | edge dist median (m) | NN median (m) | |
|---|---|---|---|---|---|---|
| Long barrows | 129 | 93.0 | 147.4 | 7.4 | 4557.0 | 1227.0 |
| Hillforts | 52 | 71.2 | 177.0 | 41.4 | 3134.0 | 4219.0 |
CHALK / GEOGRAPHY CHECK — Wiltshire long barrows vs 52 hillforts
Generated: 2026-09-09 (Europe/London)
Geology: BGS DigMapGB-625 bedrock (OGC API), Open Government Licence. Scale 1:625k — regional only.
DEM: AWS Terrain Tiles / Mapzen Terrarium z12 (~30–40 m). Prominence = elev − mean in ~1.5 km ring.
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ON CHALK
Long barrows: 93.0% (120/129)
Hillforts: 71.2% (37/52)
Both sets are overwhelmingly chalk-preferring. Shared chalk geology alone does NOT explain
the little spatial overlap — they sit on the same rock province.
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SIMILARITIES
• Both mostly on White/Grey Chalk Subgroup (BGS 625k).
• Both occupy elevated chalk country (medians ~147.4 m barrows vs ~177.0 m hillforts).
• Both have substantial easting/northing spread across Wiltshire chalklands
(barrows E-spread 51323 m / N 66778 m;
hillforts E 57170 / N 69601).
• Median nearest barrow→hillfort ~3.5 km;
hillfort→barrow ~3.1 km — cohabiting chalk, not co-sited.
• Within 2 km of a hillfort: only 24.0% of barrows.
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DIFFERENCES (niches on the same chalk)
Elevation: barrows median 147.4 m (IQR 110.6–186.8);
hillforts median 177.0 m (IQR 135.6–222.4).
Prominence: barrows median 7.4 m vs local 1.5 km mean
(mean 11.7 m);
hillforts median 41.4 m
(mean 40.4 m) — hillforts more locally proud.
Chalk edge: barrows median dist to chalk/non-chalk edge 4557 m
(IQR 1965–7462);
hillforts 3134 m
(IQR 1255–5587).
Clustering: barrows median NN 1227 m;
hillforts median NN 4219 m (hillforts more spaced).
Off-chalk: barrows non-chalk 7.0% — {'Great Oolite Group': 6, 'Mudstone, Sandstone And Limestone': 2, 'Mudstone, Siltstone And Sandstone': 1};
hillforts non-chalk 28.8% — {'Mudstone, Sandstone And Limestone': 5, 'Great Oolite Group': 4, 'Corallian Group': 3, 'Mudstone, Siltstone And Sandstone': 1, 'Lambeth Group': 1, 'Thames Group': 1}.
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VERDICT (plain)
They SHARE chalk (and high ground). They still occupy DIFFERENT niches: hillforts favour more
locally prominent / scarped positions and are more widely spaced; long barrows spread across
chalk plateaux and fringes with tighter clustering and different elev/edge profiles.
Geology explains co-occurrence on chalklands; it does not explain the small site-level overlap —
topography/prominence (and period/function) better separate the two sets.
CAVEATS
• 625k geology generalises 50k — edge distances and chalk/non-chalk at scarp toes are coarse.
• Terrarium DEM is coarse; prominence is a cheap proxy, not OS Terrain 5 / EA LiDAR.
• Superficial deposits ignored (bedrock only).
• Hillfort sample is the curated 52; barrows are 129 Wiltshire long barrows.