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The drainage-pattern of the North Welsh border was greatly modified during the Pleistocene period. Southerly diverson of the drainage caused by Welsh and Irish Sea ice-barriers effected many changes in the Alyn river system, some temporary, some permanent. The distribution of deltaic glacial gravels in the valleys indicates the formation of a chain of ice-dammed lakes linked by narrow overflow channels which notched previous water-divides. There is evidence of more than one advance of the Irish Sea ice-sheets, and the implications are extended to a wider consideration of the readvances of Welsh and Irish Sea ice in the Last Glaciation on the North Welsh border, with discussion of their dates.
