Dob's Linn

Dob's Linn

The Dob’s Linn area is where Charles Lapworth carried out his pioneering studies in the 1870s, in which he established the usefulness of graptolites for correlating different strata within the Ordovician and Silurian periods. It is also the location of the Global Boundary Stratotype Section and Point (GSSP) for the Ordovician-Silurian boundary, informally known as the ‘golden spike’.

In this area, the dominant rock type of the Southern Uplands, the Silurian Gala Greywackes, is underlain by the Upper Ordovician/Lower Silurian Moffat Shale Group. The Moffat Shale Group is exposed in a series of faulted inliers formed by imbricate thrusting, and Dob’s Linn is one such inlier. The Moffat Shale was deposited in the Iapetus Ocean over a period of 28 My, and it is composed of four main units; in chronological, and depositional, order, these are: the Glenkiln Shale, the Lower Hartfell Shale, the Upper Hartfell Shale and the Birkhill Shale. The first three of these were deposited in the Late Ordovician, while the Birkhill Shale was deposited partly in the Ordovician but mainly in the Llandovery epoch of the Silurian; the Ordovician-Silurian GSSP lies 1.6 m above its base.

The area takes its name from the Dob's Linn waterfall, which is shown in the picture. The waterfall was named after the Scottish Covenanter Halbert Dobson, who successfully hid from his English pursuers in a cave near the waterfall for several weeks in the 1690s. The society has made several field trips to the area, the last of which was a joint excursion with the Edinburgh society in April 2010.

Bill Gray.