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Both regional and localized tectonic events controlled deposition within the Wenlock and early Ludlow of SW Wales. Estuarine deposits within north-south-tending incised valleys dominate the youngest (Homerian) Gray Sandstone Group, valley incision being probably related to changing base-levels associated with Avalonia/Laurentian collision. Available accommodation space was outpaced by sediment supply, with the Red Cliff Formation (Late Ludfordian) defining a conformable transition from marine to Old Red Sandstone (ORS) deposition within the Marloes Peninsula. Sedimentation was dominated by fine-grained pedified siliciclastics, with subordinate fine-grained ephemeral sheet-flood sandstones. Local palaeocurrents indicate sediment transport from the south and west, though long-distance transport from a distant Laurentian provenance is assumed. A probable tectonically generated sequence boundary marks the base of petrographically distinctive, multi-storey pebbly sandstones of the Albion Sands Formation, deposited within the hangingwall valley of the active east-west-trending Wenall Fault. Sediment accommodation space was controlled by proximity to the tip-point of this important growth fault within the Lower ORS. Debris-flow-dominated fans, shed from both the hangingwall and footwall of the Wenall Fault, deposited the Lindsway Bay Formation, an exotic-clast conglomerate unit sourced predominantly from the south and west. It is uncertain as to whether movement along the Wenall Fault was caused by collision-related transtension, or rifting associated with the southern margin of Avalonia.

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Deposition of the Silurian (Wenlock) siliciclastic Gray Sandstone Group of south--,vest Pembrokeshire took place within littoral environments close to the palaeogeographical shelf-margin of the Welsh Basin. Sedimentation described a northerly, basinward progradation across an earlier Late Ordovician? to Aeronian rift basin. Changes in relative sea level (rsl) had profound effects on depositional environments, and five depositional sequences are recognized. During highstands of rsl, the area was influenced by wave-dominated, shallowmarine conditions. During lowstands of rsl, shelf incision and sediment bypass occurred. Associated valley fills vary in nature from high-sinuosity estuarine channels, tidal fiats and tidally-influenced, high-sinuosity fluvial channels. The last of these predominate within the youngest sequence, with abundant subaerial emergence indicators heralding the onset of true continental deposition, and the conformable transition into the overlying Old Red Sandstone Red Cliff Formation within the Marloes Peninsula. A renewal of tectonic activity ensued within the Lower Old Red Sandstone, with pebbly low-sinuosity alluvium of the Albion Sands Formation, and fanglomerates of the Lindsway Bay Formation reflecting reactivation of earlier rift-margin faults, probably within a transtensional tectonic regime.