During the climax of the last ice age, about 20,000 years ago, the Rosenheim Bogs ("Rosenheimer Stammbeckenmoore") were completely covered by the ice of the Inn River's glaciers. Only the peak of the Wendelstein Mountain and the peaks of its biggest neighbouring mountains towered above like rocky islands. The glacier scraped up the ground barried below and transported boulders and masses of earth as moraine rubble to its margin. Here it remained as a terminal moraine to the north of Wasserburg.
After the ice age, gigantic piles of moraine debris dammed up the melting waters of the glacier, causing the formation of a long 50 km lake as big as Lake Constance (Bodensee). Lake Rosenheim (Rosenheimer See) was created.
Many of the streams flowing into the lake carried fine clay particles washed out of the Alps settling down on the lake's ground. Concurrently, approximately 12,000 years ago, the "prehistoric" Inn carved it's way through the terminal moraine walls and released the lake's water.
The rain was not able to penetrate through the lake's dried up thick clay ground, causing the formation of flooded land containing riparian forests and wetland vegetation such as sedge and reed. Out of the dying vegetation, low bogs gradually raised.
The further peat development was caused by the mass spreading of dead sphagnum-mosses. During the thousands of years following, here at the foot of the Alps over the former Lake Rosenheim, one of the biggest raised bog complexes was formed, reaching a height of up to 10 m.