The igneous rocks of the Eildon Hills provide evidence of widespread volcanic activity in the Scottish Borders some 350 million years ago
Sir Walter Scott’s ‘delectable mountains’ – the Eildon Hills – have long been associated with ancient Border folklore through the work of Thomas the Rhymer and his prophecies, the Arthurian legends, and Michael Scott, the wizard, featured in Sir Walter’s great imaginative work The Lay of the Last Minstrel. Indeed, Michael is attributed with splitting the Eildon Hills into three peaks with the help of an idle devilish spirit. But perhaps the geological explanation is a little more plausible – one of weathering and erosion by water and ice leaving the harder igneous rocks standing proud of the surrounding land.
Overlooking the beautiful valley of the River Tweed, with Melrose lying immediately below, the three conical peaks of the Eildon Hills (North Hill, 404 m above sea-level, Mid Hill, 422 m and Wester Hill, 371 m) evoke an exciting geological and historical past. The Eildons, known by the Romans as Trimontium (the name also applied to the major fort east of the present village of Newstead) have distinctive volcanic shapes. In fact, the underlying rocks are not the original surface of a volcano but instead are the eroded remnants of a suite of igneous trachytic and rhyolitic sills – places where magma has squeezed in between the layers of sedimentary rocks and solidified, creating a layer of igneous rock. The sills in the Eildons are around 350 million years old (Early Carboniferous), and intruded into the uppermost Devonian sedimentary rock. Compositionally, the silica content of trachytes and rhyolites is much greater than that of basalts. A small prominence, called Little Hill, lying between Wester Hill and Mid Hill is the remnant of a volcanic plug now filled with agglomerate and basalt – this was probably a conduit to a small volcano.
On lower ground, to the northwest of the Eildons closer to Melrose and near the Borders General Hospital, another small hill is underlain by a large vent called the Chiefswood Vent. This is filled with rock formed from fragments of sedimentary rock including Devonian sandstones and Silurian greywacke sandstones, together with fragments of trachyte and rhyolite. At the time the vent was created, the area might have been low-lying swamp, with volcanoes erupting and creating new land. The rock in the vent originated from the explosive interaction of silica-rich magmas with surface waters. The resulting brown to greenish-grey vent rock was quarried extensively in the 12th century for use in the earliest parts of Melrose Abbey. Examples can also be seen in the walls of some of the older buildings in Melrose. Local warm-coloured Devonian sandstones were also used as building stone extensively for the Abbey and in this part of the Tweed valley.
Text contributed by Andrew McMillan
Find out more
There are many paths on the Eildon Hills including St Cuthbert’s Way which traverses the hills through the col between Mid Hill and North Hill. One of the easiest approaches to the hills is from the west. Limited road-side car parking is available next to Bowdenmoor Reservoir off the B6359. From here a traverse of the hills is possible, firstly observing en route reddened greywacke sandstones in a small quarry to the east of Bowdenmoor Reservoir, and then taking in the Little Hill vent where both intrusive basalt and vent rocks can be seen. Wester Hill and the steep slopes of Mid Hill are made up of pinkish felsite (a very fine-grained rhyolite composed of quartz and feldspar). The summit of Mid Hill and the whole of North Hill are made from sheets of trachyte (an igneous volcanic rock rich in silica and potassium feldspar).
Clarkson, E, and Upton, B. 2010. Death of an Ocean – a Geological Borders Ballad (Edinburgh: Dunedin Academic Press), 176-181
McAdam, A D, Clarkson, E N K, and Stone, P. (editors). 1992 Scottish Borders Geology – an excursion guide (Edinburgh: Scottish Academic Press), 101-111 (Building Stones of the Borders Abbeys by I T Bunyan) and 112-120 (Eildon Hills by R J A Eckford and W Manson).
Upton, B. 2015. Volcanoes and the Making of Scotland (Edinburgh: Dunedin Academic Press), 165-166
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