A great difference, however, is to be remarked between the coasts of the North Sea and those of the Baltic. On the former, where the sea has broken up the ranges of dunes formed in bygone times, and divided them into separate islands, the mainland has to be protected by massive dikes, while the Frisian Islands are being gradually washed away by the waters.
The chain of the Frisian Islands marks the outer fringe of the former continental coast-line, and is separated from the mainland by shallows, known as Wadden or Watten, answering to the maria vadosa of the Romans.
In the south the northernmost of the North Frisian Islands (Fanb) is Danish.
With the exception of Wangeroog, which belongs to the grand duchy of Oldenburg, the East Frisian Islands belong to Prussia.
About the year 1250 the area of the North Frisian Islands was estimated at 1065 sq.
The West Frisian Islands belong to the kingdom of the Netherlands, and embrace Texel or Tessel (71 sq.