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When the last great ice age drew to an end, glaciers slowly retreated, revealing characteristic U-shaped valleys, and great banks of terminal moraine formed dams that turned rivers of meltwater into the lakes for which Switzerland is also known. When the glacier that had filled the Surselva valley on the east side of the Oberalp Pass withdrew, a vast section of mountain collapsed into the valley below where Flims now stands, blocking the river for a length of something like 15km. When the pressure of water finally broke through the blockage, a wall of water swept downvalley causing much devastation.

In the warm period of 2000–4000 years ago, much less ice covered the Alps than there is today. Relics found near the 3301m summit of the Theodulpass show that it was snow-free in Roman times, and the tongues of 1st-century glaciers were then at least 300m higher than they are now. Throughout the Middle Ages, a number of passes that are covered by glaciers today were free from snow and ice and were in regular use.