evidence: As already remarked, the coast-line of Holland breaks up into a series of islands at its northern and southern extremities. The principal sea-inlets in the north are the Texel Gat or Marsdiep and the Vlie, which lead past the chain of the Frisian Islands into the large inland sea or gulf called the Zuider Zee, and the Wadden or "shallows," which extend along the shores of Friesland and Groningen as far as the Dollart and the mouth of the Ems. The inland sea-board thus formed consists of low coasts of sea-clay protected by dikes, and of some high diluvial strata which rise far enough above the level of the sea to make dikes unnecessary, as in the case of the Gooi hills between Naarden and the Eem, the Veluwe hills between Nykerk and Elburg, and the steep cliffs of the Gaasterland between Oude Mirdum and Stavoren. The Dollart was formed in 1277 by the inundation of the Ems basin, more than thirty villages being destroyed at once. The Zuider Zee and the bay in the Frisian coast known as the Lauwers Zee also gradually came into existence in the 13th century.