As part of the Schleswig-Holstein Morainic Uplands (Ger. "Schleswig-Holsteinisches Moränenhügelland"), that were formed during the Weichselian glaciation, these peninsulas are hilly and dotted with several glacial lakes. The Anglian glacial lakes form the North Anglian Lake Group (Ger. "Nordangeliter Seengruppe"). The River Treene (Dan. "Trenen") with its main headstream Bondenau (Dan. "Bondeåen") rises in Anglia. Although rising on the Anglian Peninsula in the Baltic Sea, the Treene flows towards the North Sea, being the main tributary of the River Eider (Dan. "Ejderen"), the river that constituted the Southern border of the Danish Realm for a very long time. The northernmost part of Anglia is formed by the Holnis (Dan. "Holnæs") Peninsula that protrudes into the Flensburg Firth.