Irish Sea origin during the more recent Devensian ice age, a combination of the two ice-masses reaching their maximum extent over the county around 18,000 years ago when thicknesses over the plain were perhaps up to 400m. At this point Irish Sea Ice having advanced south across the Cheshire Plain reached as far as Wenlock Edge and the Wolverhampton area. Ice tongues from the Welsh Icesheet reached down the Teme, Clun and Onny valleys to the south of Long Mynd and Stiperstones though the higher ground remained ice-free. The Severn and Rea valleys were also occupied by Welsh ice moving either side of Long Mountain. The so-called Ellesmere and Welsh readvances occurred around 16,000 years ago but began retreating 1000 years later. All ice had gone by 11,000 years ago. A northwest-southeast aligned boundary between the two ice-masses is recognised between Ellesmere and Oswestry, its position changing over time. Southeast Shropshire remained unaffected by glaciers though as with other areas rising above the ice surface, the landscape was affected by intense periglaciation. Till is up to 100m thick in parts of north Shropshire. Erratics within the till can be traced back to granite outcrops in southern Scotland and the Lake District as well as sources which are closer. Meltwater channels Glacial meltwater flowing beneath, beside or down-valley of the glaciers cut several gorges, the most significant of which is the Severn Gorge at Ironbridge. Its formation was traditionally ascribed to the overflowing of a hypothetical proglacial lake termed Lake Lapworth deemed to have covered much of northern Shropshire and adjacent parts of Staffordshire. The currently accepted explanation involves the proven existence of a major subglacial trench (now infilled by later sediments) beneath the ice sheet west of Shrewsbury and leading towards the gorge. Large quantities of water under considerable pressure were able to then move uphill and over the pre-existing watershed east of Buildwas, powerfully eroding it in the process and giving rise to the modern gorge feature, possibly during multiple glacial phases. The deeply incised Marrington Dingle conveys the Camlad north from Churchstoke and originates as the outflow from a glacial lake in the Churchstoke/Snead area. The Plowden gorge permits the Onny to flow east around the southern end of the Long Mynd and was also initiated by meltwater. Sediment-filled valleys are recorded from the Telford area, one of which, the Lightmoor Channel, runs southeast from Madeley on a