The Clwydian Hills are formed from an upstanding block of deep sea sediments formed during the Silurian period as debris slurries originating on the nearby continental shelf. The older mudstones and siltstones of the Nantglyn Flags Formation form parts of the west-facing scarp slope and the overlying Elwy Formation which consists of mudstones and siltstones deposited in deep marine conditions with numerous sandstone beds form most of the higher ground. Both formations are of Ludlovian age. The range's rocks are intensely faulted; the major Vale of Clwyd Fault is responsible for the impressive west-facing scarp of the Clwydian Range. It downthrows the rocks to the west and separates the younger Carboniferous and Permo-Triassic rocks of the Vale of Clwyd from those of the hills.