The Reesholm peninsula took on its current shape during the last Weichselian glaciation, when a wide tongue of ice advanced to the western end of the Schlei basin. This is how the Schlei channel was formed in the post-glacial period. The higher part of Reesholm remained as a flat moraine ridge.
Since 1976, Reesholm and the island of Hestholm in the Große Breite have been designated a nature reserve with the code number 87, covering 120 hectares.
Large parts of the meadowland on Reesholm are only just half a meter above sea level and are therefore exposed to flooding. This is how a botanically valuable salt marsh landscape developed.
Sand hooks are forming at the southern tip, which are caused by coastal collapse on the steep bank on the west coast of the peninsula, which is three meters above sea level.