An Ashgill to Llandovery offshore marine sequence in Mid-Wales is punctuated by a shallow marine sequence (the Scrach Formation) attributed to Late Ordovician glacio-eustatic regression. The base of the regressive sequence is gradational and suggests progressive fall in sealevel during mid-Hirnantian (late Ashgill) time. An abrupt top to the Scrach Formation marks a rapid sealevel rise, probably within the persculptus Zone (late Hirnantian). Interbedded lenticular thin sands and muds dominate the Scrach Formation and represent shallow shelf environments, with evidence of NW-directed tidal ebb flow and storm currents. Sporadic medium-bedded sand sheets represent the strom events. Locally, lenticular thick-bedded sand units embedded in the fine facies probably represent barrier facies in an area of high clastic input.

The Scrach Formation thins SE into the present NE-striking Myddfai steep belt. This is underlain by NE-striking faults that actively controlled late Ordovician to early Silurian sedimentation. The faults probably defined an emergent but low relief coast during the glacial regression. Offshore, to the NW, the Scrach passed into parallel laminated mudstones, then across a fault-influenced slope (the present Tywi lineament) into coarse clastic sediments deposited by gravity flows.

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