
Since the beginning of the Quaternary (2.58 million years ago) glacial periods have shaped the landscape. In a progression phase, the glaciers move forward and dig a hole in the ground. Sometimes they meet harder rocks which they "bypass". At their moving, they leave behind valleys. The hard rocks left behind can form mountain glaciers: hills that make the valley higher and much more narrow. In history, mountain glaciers were strategic venues for homeland defence, like the Fort Queyras.