A provenance study of rocks from the Silurian inliers and Siluro-Devonian Lanark Group sandstones of the southern Midland Valley has shown that they are petrographically and compositionally similar, and were derived from the same source terrane. The clast content of these rocks indicates that before widespread Lower Devonian calc-alkaline volcanism, this source terrane included volcanic and hypabyssal igneous rocks that appear to have been associated with a wacke sandstone-dominated sedimentary sequence and a granitic igneous suite. The possibility that the source rocks for these sequences are within the Midland Valley Terrane, but now not exposed, is discussed. Minor differences in sandstone composition between the Carmichael, Eastfield and North Esk inliers are interpreted as reflecting either slight differences in the source or deposition within individual sub-basins. Comparable regional variations within the Swanshaw Sandstone Formation suggest that sub-basin architecture continued to influence sediment dispersal patterns during the deposition of the basal Lanark Group. The onset of Lower Devonian calc-alkaline volcanism coincided with a major change in sandstone composition within the Lanark Group and accompanied the replacement of the relatively small Silurian sub-basins by a single larger basin.