In late-Precambrian and early Palaeozoic times, the Clyde Sea Area lay at the junction of a major land mass and the Iapetus Ocean. Closure of the Ocean on its northwestern margin corresponded broadly with initiation of the Highland Boundary and Southern Uplands Faults and formation of southwestnortheast oriented basins of continental sedimentation. Northwest-southeast fractures of the Clyde Belt, along which the Northeast Arran and Southwest Arran troughs of the firth are emplaced, developed across the southwest-northeast Caledonoid grain in Carboniferous-Permian times. The firth and estuary began to assume their distinctive forms in Tertiary times. The area experienced several major glaciations during the Quaternary Period. Late Devensian effects are most distinct: overdeepening of the firth to depths exceeding — 160mOD; deposition of till and meltwater sediments up to 80 m in thickness. Clyde Beds, fossiliferous clays and silty sands up to 15 m in thickness, accumulated c. 13,150–10,000 BP in sub-arctic marine waters in the firth, estuary and Paisley embayment, and in the Lomond basin prior to the Loch Lomond Stadial. The Holocene marine transgression, which produced further modifications in shoreline position, e.g. in Ayrshire, the Paisley embayment and the Lomond basin, was followed by marine regression due to isostatic land rebound.