The source of the river Ammer is in Graswang Valley. It drains a part of the Ammer Mountain Range, flows into the Lake Ammer north of Weilheim and is called the river Amper when it meets the river Isar in Moosburg. The Ammer Valley is one of the most beautiful natural river valleys in the northern Limestone Alps. The entire river is 185 km long.
During the Wurm Ice Age, around 120,000 years ago, the Ammer meandered its way through the Ammer-Loisach uplands after leaving the Alps. The splendid Ammer Gorge (Ammerschlucht) between Altenau and Peißenberg was formed in this manner. Here, approximately 15 km from its source, the Ammer created this impressive gap in the rock, the Scheibum, which measures 600 m in length, is up to 80 m deep and has been under protection since as early as 1949.