Schiehallion: the mountain that weighed the Earth

This superb quartzite mountain was chosen in 1774 as the site of a famous experiment to weigh the Earth; today it is an unrivalled viewpoint

View of Schiehallion from Loch Rannoch to the west. Image: Carol Pudsey.

Six to seven hundred million years ago the area that now forms the Grampian Highlands was a shallow sea. Layer upon layer of sediment was formed from mud eroded from the land, white quartz sands, and limy deposits. As these layers were buried and compressed, they became mudstone, sandstone and limestone. At that time Scotland was at the edge of a continent which included North America, separated by the deep Iapetus Ocean from the rest of Europe. Plate tectonic movements closed this ocean 470 to 430 million years ago, subjecting the buried rocks to great heat and pressure. Mudstone became schist and slate; sandstone became very hard, creamy-white quartzite; all were folded and fractured in complex patterns. At the end of these earth movements the rocks were uplifted to form the Caledonian Mountains.

Perhaps one of the most amazing things about Schiehallion is that this folding and fracturing of the rock layers is so pronounced in the area around the mountain that the mountain itself is almost ‘upside-down’! In other words, the older rock layers can be found at the top of the mountain and the younger layers at the bottom.

Several different rock types can be seen on the main path up Schiehallion,. You may come across a limestone pavement with water-worn fissures and potholes. Locally, these limey soils support unusual plants. The same rock can be seen in quarries and a restored 19th century lime kiln at Tomphubil. . Most of the ridge is grey to white quartzite, with current bedding in a few places and an occasional band of pinkish brown microdiorite. In the Tempar Burn to the northwest is the famous Schiehallion Boulder bed, an ancient glacial tillite.

During the Ice Ages of the last two million years, glaciers flowed east from Rannoch Moor, carving the hard quartzites of Schiehallion into a streamlined ridge and digging deep valleys on either side. The ice has carried blocks of granite and schist, and dropped them all the way along the east ridge, up to a height of about 980m. From there to the summit the path goes over bedrock with only local quartzite boulders.

The experiment of 1774 to weigh the Earth involved measuring the deflection of a plumb line resulting from the gravitational pull of a nearby mountain. Schiehallion was considered the ideal mountain, due to its isolation and almost symmetrical shape. The tiny deflection of a plumb-line from the vertical must be measured relative to the fixed background of the stars, which requires extremely careful measurements on either side of the mountain. The mass of the mountain can be worked out from its volume and the density of its rocks. These values can be used to find the gravitational pull of the Earth, and thus its mass.

Text contributed by Carol Pudsey

Find out more
Grid references for places of interest:
limestone pavement NN74605464
From observatories on the north and south sides of the mountain, Sir Nevil Maskelyne, the Astronomer Royal, made precise observations of stars. His two observatory sites can still be seen at NN72005412 (725m) and NN72105545 (635m). To find the volume of the mountain, a team of surveyors mapped its shape and one of their observation cairns is still there on the east ridge (NN72695457).

Treagus, J.E, Tanner, P.W.G, Thomas, P.R., Scott, R.A and Stephenson, D. 2013. The Dalradian rocks of the central Grampian Highlands of Scotland. Proceedings of the Geologists’ Association 124, 148–214, sections 17 and 18.

At the summit of Schiehallion, the layers of quartzite are vertical and stand out as rocky ribs running east-west. There is a fine view along the valley of Lochs Tummel and Rannoch, carved out by glaciers. Image: Carol Pudsey.

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