Barrier islands developed on the southeastern flanks of a volcanic terrain during the Lower Silurian transgression of southwest Wales. The barriers are preserved in transgressive sequences overlying basalts and comprising from base upwards: lagoon[RIGHTWARDS ARROW]barrier island[RIGHTWARDS ARROW]offshore marine sediments. The thickness of the barrier island sediments varies from 5 m to 28 m. Comparison with modern barriers suggests that the thin sequences result from narrow (<2 km), steadily transgressing barrier islands, whereas the thicker sequences represent broad (2–4 km), slowly transgressing forms. In one case the barrier became narrower as the rate of migration accelerated in response to decreased fluviatile sediment supply caused by rising sea-level. Despite the high preservation potential of inlet fill deposits, the latter are generally absent in these Silurian barriers because inlet migration was slow compared with the rate of barrier retreat. Possibly much shell material was dissolved during early diagenesis.