As late as the 12th and 13th centuries each of these islands consisted of several smaller islands, many of whose names are still preserved in the fertile polders which have taken their place.
Some polders also have a winter peil as a precaution against the increased fall of water in that season.
In time of drought the water in the canals and boezems is allowed to run back into the polders, and so serve a double purpose as water-reservoirs.
The area south of this would be divided into four polders, with reservation, however, of a lake, Yselmeer, in the centre, whence branches would run to Ysel and the Zwolsche Diep, to Amsterdam, and, by sluices near Wieringen, to the northern part of the sea.
The four polders with their areas of fertile soil would be: (I) North-west polder, area 53,599 acres; fertile soil, 46,189 acres.