Allars Mill, Jedburgh

An unconformity between Silurian greywackes and Upper Old Red Sandstone beds is found on a river cliff above the Jed Water. While the site is degraded by scree and vegetation cover, it is a classic site in the history of geological ideas, having been visited and described by James Hutton.

The famous Allars Mill unconformity between Upper Old Red Sandstone (Stratheden and Inverclyde Groups – undifferentiated) and underlying Silurian greywackes can be found 0.5 km south of the centre of Jedburgh next to the Jed Water. The river cliff lies below the A68. While it is now somewhat covered by slipped material so that the line of the unconformity cannot be seen, Sir John Clerk made a drawing at the time reproduced in the account by McAdam, A.D., Clarkson, E.N.K. & Stone, P., 1993 (see reference below) which explains the geology of the site.
At present, about 2 m of bedded Upper Old Red Sandstone mudstones and siltstones can be seen at the top of the river cliff below the road and exposures of red silts and clays, mostly covered by vegetation, can be seen for some metres below that, although the lower areas could be slumped soil and mud. Greywackes, probably belonging to the Hawick Group (Silurian: Llandovery-Wenlock), can be seen in the river bed upstream for some distance from the bridge which gives access to Allars Mill. At the unconformity site they extend to at least 4 m above the river bed. Structures cannot be distinguished clearly as the exposures are largely covered with branches and ivy, but at river level the greywackes appear to be near-vertical and have thin to medium-bedding. There is no sign in the cliff of the greywacke breccia which Clerk shows as lying between the vertical bedding and the overlying red mudstones and siltstones.