Complex sentence: It is a vast plain comprising large brine lagoons or "étangs", cut off from the sea by sandbars and encircled by reed-covered marshes which are in turn surrounded by a large cultivated area.
Split into simpler sentences: It is a vast plain comprising large brine lagoons or "étangs", cut off from the sea by sandbars and encircled by reed-covered marshes. These are in turn surrounded by a large cultivated area.