The lay-out of the soil map of the Belgian maritime plain (1947-1953) has made it possible to correct our present-day ideas about the formation of our maritime polders. The main fact in this formation is the Dunkirk transgression, which comprises three phases: the Dunkirk 1 transgression (2nd century B.C. to 1st century A.D.), the Dunkirk 2 transgression (4th to 8th century) and the Dunkirk 3 transgression, subdivised in two subsidiary phases : the Dunkirk 3A transgressive phase (11th century) and the Dunkirk 3B transgressive phase (12th century). In the course of each of these transgressive phases sediment layers have been deposited, which constitute the present surface of the polder plain. According to their geological constitution, five typical geographical areas in the polder plain can be distinguished: the ancient area, the intermediate area, the recent area, the historical Ostend polders and the drained marshes.