Complex sentence: Built in the Iron age, the purpose of Fogous (derived from the Cornish word for cave) is not known, however, it has been speculated that they could have been used for food storage or for religious ceremonies.
Split into simpler sentences: Built in the Iron age, the purpose of Fogous (derived from the Cornish word for cave) is not known. It has been speculated, however, that they could have been used for food storage or for religious ceremonies.