 These were formed approximately 340 million years ago as a reef fringing a shallow lagoon, with deeper water beyond.The presence of a small outcrop of fossiliferous rock (known as 'beach beds') at the base of Winnats Pass, close to Speedwell Cavern, suggests that a contemporary underwater cleft or canyon once existed within the active reef which caused the build up of shelly and crinoidal remains at its base.All these sediments were subsequently buried together under Namurian sandstones and shales in the subsequent Upper Carboniferous period.