Complex sentence: The island is separated by a 16 km stretch of sea from the Argentario promontory: mainly mountainous, it consists almost entirely of granites culminating in the Poggio della Pagana (496 m).
Split into simpler sentences: The island is separated by a 16 km stretch of sea from the promontory of Monte Argentario. Mainly mountainous, it consists almost entirely of granites culminating in the Poggio della Pagana (496 m).