The Frisians migrated to North Frisia from the South in two waves. During the 8th century A.D. they mostly settled on the islands Heligoland, Sylt, Föhr, Amrum and presumably also in parts of the Eiderstedt peninsula. The coastal marshlands of the mainland were settled in a second wave and after a series of storm surges the Frisians also used to settle on the higher inland geest. While the marshland and its bogs had to be drained, the higher geestland cores of the islands were in turn mostly barren and needed fertilisation before a proper agriculture could be established.