sentences:  Rocks dating from the Permian period occur in North Wales underlying the Vale of Clwyd and extend into the northeastern fringes of the country from the larger bodies of such rock in Cheshire and Shropshire.The Permo-Triassic rocks in the northeast of Wales are largely concealed by recent material deposited by rivers and the Devensian icesheet.Thicker sequences of Permo-Triassic rock are known to underlie the Bristol Channel, Cardigan Bay and the Irish Sea off the North Wales coast.