There are five geomorphological units in the Adriatic: the Northern Adriatic (up to 100 meters deep); the North Adriatic islands area protected against sediments filling it in by outer islands (pre-Holocene karst relief); the Middle Adriatic islands area (large Dalmatian islands); the Middle Adriatic (characterized by the Middle Adriatic Depression); and the Southern Adriatic consisting of a coastal shelf and the Southern Adriatic Depression.