Complex sentence: They consist of an octagonal concrete base that measures 3,850 mm across with a channel running west to east across the middle; on the southern edge of the octagon is a raised area of concrete which is 225 mm higher than the rest of the base.
Split into simpler sentences: They consist of an octagonal concrete base that measures 3,850mm across, with a channel running west to east across the middle. On the southern edge of the octagon is a raised area of concrete, which is 225mm higher than the rest of the base.