In the southern part of the Scottish Midland Valley Lower Old Red Sandstone sedimentary and volcanic strata of the Lanark Group unconformably overlie inliers of largely marine Silurian rocks. The oldest of the four formations of the Lanark Group is the Greywacke Conglomerate Formation which contains clasts predominantly of greywacke, with subordinate volcanic rocks, cherts and limestones. The greywackes are known to have been derived from a cryptic source which lay to the south and east of the Midland Valley. Contrary to earlier, Silurian, age assessments of limestone clasts from the Greywacke Conglomerate Formation of the Pentland Hills, a conodont faunule described herein belongs in the uppermost Llanvirn to lower Caradoc P. anserinus Biozone. There are no Ordovician rocks exposed within the Pentland Hills inlier, and thus our new age data suggest that along with the other clasts, the limestones in the Greywacke Conglomerate are exotic to the Pentlands Sub-basin. The source area had a cover succession that included mid-Ordovician shallow marine carbonate and flysch.