In 1940 Professor O. T. Jones published in the Geological Magazine significant observations on certain of the Lower Palaeozoic contacts in north Pembrokeshire. His detailed inspection of the Solva-Caerfai contact at Caerfai Bay, and the Arenig-Lingula Flag junction at Whitesand Bay, revealed erosional and depositional features which clearly established hitherto debatable lines of unconformity. In addition, his results have aroused much needed assiduity in the interpretation and inspection of formational breaks in the Lower Palaeozoic rocks. Shearing and structural deformities, as well as the emplacement of cleavage, frequently mask such erosional and depositional features but, as Professor Jones has demonstrated, vestiges of them sometimes remain.