The Rocks Beneath: An Interactive Geological Map of the British Isles and the World, in the Colours of the 1969 Survey

I describe an open, interactive geological map of the British Isles and the world, coloured with a palette sampled from the index of the fifth edition (1969) of the Geological Map of the British Islands, and use it to measure what the islands are made of. The British Geological Survey's 1:625,000 bedrock map (243 units) and Geological Survey Ireland's 1:500,000 map (83 units) are drawn onto a common Web Mercator grid of about 175 m, with Macrostrat's generalised world map (4,639 units) at about 5 km. Each unit is given an age range, a period, a rock type and one of 37 colour classes keyed to the rock groups the 1969 map used. Carboniferous rocks cover more of Great Britain than any other period (17.5%), and 54.7% of the Republic of Ireland; 16.5% of Great Britain is Precambrian. Along a line from Scafell Pike to London the age of the bedrock falls almost steadily (Spearman's rho = −0.92), and across England and Wales the south-eastward tilt of the strata gives rho = −0.76 between position and age. The Coal Measures cover 5.1% of Great Britain but carry 18.5% of its cities and towns. Superficial deposits hide the bedrock over 54.0% of Great Britain, till alone over 29.3%. On the world map 23.8% of the dated, ice-free land is Precambrian, from 86.2% of Fennoscandia to under 10% of China and the United States. The maps are generalised and the world map mixes sources of very different detail; the results describe broad patterns, not individual outcrops. Interactive version: https://tristan-hotham.github.io/rocks-beneath/. Data and code: https://github.com/Tristan-Hotham/rocks-beneath. Blog post: https://blog.tristanhotham.com/2026/10/28/rocks-beneath/. Contains British Geological Survey materials © UKRI 2026 (BGS Geology 625k, DiGMapGB-625, Open Government Licence). Contains Irish Public Sector Data (Geological Survey Ireland) licensed under a Creative Commons Attribution 4.0 International (CC BY 4.0) licence. Contains UK public sector information (Geological Survey of Northern Ireland) licensed under the UK Open Government Licence v3.0. World geology from Macrostrat (macrostrat.org), CC BY 4.0. Contains OS data © Crown copyright and database right 2026 (OGL v3) and GeoNames data (CC BY 4.0). Colours were sampled from the index of the Geological Map of the British Islands, 5th edition (1969); the map itself is not reproduced. Derived data, paper and figures: CC BY 4.0. Code: MIT.