 Coal measures appear in the Radstock district , and surrounding Somerset Coalfield ( largely concealed by Triassic and newer rocks ) . There are two series of coal @-@ bearing sandstones and shales separated by Pennant Sandstone . Locally the beds are folded and faulted . There were mines in the Radstock and Nailsea areas but these have closed . This was one of the first areas in the world to undergo systematic geological study and mapping by John Strachey and William Smith in the 18th century . They observed the rock layers , or strata , which led Smith to the creation of a testable hypothesis , which he termed the Principle of Faunal Succession . 
