Complex sentence: During much of the most recent ice ages, from 1.8 million years ago, the sea level in the Severn Estuary was some below the current level and Flat Holm was joined to the Somerset coast as an extremity of the Mendip Hills.
Split into simpler sentences: During much of the most recent ice ages, from 1.8 million years ago, the sea level in the Severn Estuary was some below the current level. During the ice ages, Flat Holm was joined to the Somerset coast as an extremity of the Mendip Hills.