 The Quantock Hills are largely formed by rocks of the Devonian Period , which consist of sediments originally laid down under a shallow sea and slowly compressed into solid rock . In the higher north western areas older Early Devonian rocks , known as Hangman Grits , predominate , and can be seen in the exposed rock at West Quantoxhead quarry , which were worked for road building . Further south there are newer Middle and Late Devonian rocks , known as Ilfracombe beds and Morte Slates . These include sandstone and limestone , which have been quarried near Aisholt . At Great Holwell , south of Aisholt , there is a limestone cave , which is the only one in the Devonian limestone of North Devon and West Somerset . The lower fringes around the hills are composed of younger rocks of the Triassic period , these are known as New Red Sandstone rocks which represent the deposits of large river systems that crossed a desert plain , and often contain irregular masses or veins of gypsum , which was worked on the foreshore at Watchet . The scarp is to the west with a dip slope to the east . The west side is cut by combes with broad valleys on the east . The hill tops are open heathland with woods on the slopes . 
