Complex sentence: The Great Orme is a peninsula made mostly of limestone and dolomite, formed during the Early Carboniferous part of the Earth's geological history--most of the Great Orme's rocks are 339--326 million years old.
Split into simpler sentences: The Great Orme is a peninsula made mostly of limestone and dolomite, formed during the Early Carboniferous part of the Earth's geological history. Most of the Great Orme's rocks are between 339 and 326 million years old.