 The main geological component of the Mendips is Carboniferous Limestone . It represents the remnants of a much higher range of hills that existed hundreds of millions years ago . This has allowed the formation of features such as Cheddar Gorge , Ebbor Gorge and Burrington Combe . There are a wide variety of caves and <unk> holes caused by dissolution of the rock by water . Further east there are Silurian volcanoes , Carboniferous Limestone outcrops , Variscan thrust tectonics , Permo @-@ Triassic conglomerates , sediment @-@ filled fissures , a classic unconformity , Jurassic clays and limestones , Cretaceous Greensand and chalk topped with Tertiary remnants including <unk> Stones . These sediments have yielded a fairly rich fossil fauna of brachiopods and trilobites indicating that they were deposited in a shallow marine sea into which the lavas were extruded . The rocks are quarried at Moons Hill near Stoke St Michael for aggregate . 
