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Perth & Kinross, Scotland


Schiehallion, 1083m above sea level, lies in the Grampian Mountains of Perthshire. Its simple, sail-like profile and isolation from adjacent hills makes it a dramatic sight in the central Highlands – and is why it hosted one of Earth Sciences’ most important early geophysical experiments – to find the mean density of the Earth. 

SchiehallionIt was here in 1774 that Nevil Maskelyne made a series of astoundingly accurate plumb-line experiments that measured the gravitational attraction of the mountain. By knowing the density of rocks that made up the mountain, and by being able to survey its simple shape, the discrepancy between the measured plumb line defection and the amount expected from the mass of the mountain, Maskelyne, and his associate, Charles Hutton, estimated the average density of the Earth of about 4500 kg.m-3. This compares with modern estimates of 5515 kg.m-3. 

However, it was the polymath John Playfair (see Siccar Point) who in 1811 re-interpreted Maskelyne’s measurements and recalculated the earth’s mean density as 4560-4870 kg.m-3 by using more complex models of Schiehallion’s internal geological structure. In doing so, Playfair became the first person to link the gravity field to local geological structure. 

Schiehallion therefore lies at the beginning of the use of potential field data to understand crustal structure – a fundamental technique in Earth Sciences.

Text: Professor Rob Butler

100 Great Geosites

Further Reading

Smallwood, J. 2009. John Playfair on Schiehallion, 1801–1811. LEWIS, C. L. E. & KNELL, S. J. (eds) The Making of the Geological Society of London.
The Geological Society, London, Special Publications, 317, 279–297.
Images (Top to bottom):
  • Schiehallion viewed across the River Tay ©Andrew2606 (Source Wikimedia Commons) Creative Commons Attribution-Share Alike 3.0 Unported
  • The symmetrical ridge of Schiehallion from Loch Rannoch © Anne Burgess (Source Creative Commons Attribution-ShareAlike 2.0 license