Complex sentence: The island and coastal ranges are thought to have been thrust up as blocks with subsidence between them to form the coastal plain with the summit level of the island being older dissected surface that has been uplifted to approximately 1km or more above sea level.
Split into simpler sentences: The island and coastal ranges are thought to have been thrust up as blocks with subsidence between them to form the coastal plain. The summit level of the island is thought to be an older dissected surface than has been uplifted to approximately 1km or more above sea level.