Dob's Linn

A steep little gorge of NE Dumfries and Galloway, close to the boundary with the Scottish Borders, Dob's Linn is located on the west side of the A708 road, 4 miles (6.5 km) southwest of Tibbie Shiels Inn on the southern end of St Mary's Loch. Dob's Linn is a Site of Special Scientific Interest (SSSI) owing to its considerable geological importance as the Global Boundary Stratotype Section and Point (GSSP), or internationally-agreed reference section, for the boundary between the Ordovician and Silurian periods of the geological time scale.

The rocks comprise black shale deposited in the deep waters of the Iapetus Ocean, that once separated England from Scotland, which were then thrust upwards with the closure of that ocean around 425 million years ago. The sediments contain graptolite fossils which were used by Charles Lapworth in 1878 to decode the geological structure of the area. He established them as a means of correlating Ordovician and Silurian rocks across Britain, a system which was extended worldwide by others.

The same fossils are found in similar rocks in Newfoundland (Canada) suggesting Scotland and Canada were part of the same supercontinent before the opening of the Atlantic Ocean around 140 million years ago.

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