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Dob's Linn

Dob's Linn near Moffat is one of the sites where, in the 1860's and 1870's, Charles Lapworth (1842 - 1920) collected and studied the fossilised graptolites of the Southern Uplands. Lapworth was the first scientist to recognise the well-defined change in graptolite assemblages throughout the sequence of exposed shales. The results of his work, published in 1878, established the importance of using graptolites to understand the stratigraphic sequence and structural complexities of the Southern Uplands. Following this important work, graptolites have become one of the most important zone fossils for correlating Ordovician and Silurian rocks the world over. In 1985, Dob's Linn was formally announced as the stratotype for the Ordovician - Silurian boundary.

Dob's Linn lies in the central Southern Uplands. The Southern Uplands is mostly composed of Silurian greywackes (the Gala Greywacke), but the Upper Ordovician/Lower Silurian Moffat Shale Group is exposed in E-W faulted inliers (formed by imbricate thrusting). One of these inliers is exposed at Dob's Linn.

The Moffat Shale Group is composed of four formations:

• Glenkiln Shale (Llandeilo - Caradoc)
• Lower Hartfell Shale (Caradoc - Ashgill)
• Upper Hartfell Shale (Caradoc - Ashgill)
• Birkhill Shale (Ashgill - Llandovery, i.e. Ordovician - Silurian)

Dob's Linn SSSI the location of the internationally agreed boundary stratotype between the Ordovician and Silurian systems.
Image provided by Scottish Natural Heritage.

The fossils found include:

• Glenkiln Shale (Nemagraptus gracilis, Climacograptus peltifer)
• Lower Hartfell Shale (Climacograptus wilsoni, Dicranograptus clingani, Pleurograptus linearis)
• Upper Hartfell Shale (Dicellograptus complanatus, Dicellograptus anceps, Climacograptus? extraordinarius)
• Birkhill Shale (Glyptograptus persculptus, Coronograptus gregarius, Rastrites maximus)

The rocks here are a sequence of black shales and grey/black mudstones, with the black shales being the most fossiliferous. The entire sequence is estimated to be about 100m thick and was deposited in the deep waters of the Iapetus Ocean.

Further reading:

Lawson, J.D. & Weedon, D.S. 1992. Geological Excursions around Glasgow & Girvan. Geological Society of Glasgow.

Webb, B.C., Rushton, A.W.A. & White, D.E. 1993. Moffatdale and the Upper Ettrick Valley. Description of the solid geology of parts of 1:25,000 sheets NT 10, 11, 20 and 21. British Geological Survey (Her Majesty's Stationery Office, London).