Following the expansion and retreat of the glaciers, northern Italy had taken on the appearance we know, with an elevated mountain chain (the Alps), valleys cut by rivers and streams, a series of lakes all arranged perpendicularly to the axis of the Alpine chain and a valley border composed of gentle, fertile moraine hills. But, given its depth in the central area, glaciers cannot have been solely responsible for its formation. Earlier, in fact, it was the rivers that carried out their erosive action, carving narrow and deep valleys that would provide guidance for the glacial masses in later expansion. This happened 7-8 million years ago when the narrow passage between the Atlantic and the Mediterranean (Strait of Gibraltar) closed, making the latter a closed basin with progressive evaporation of its waters.

When the connection with the ocean was re-established, at first the sea inundated previously emerged land, then receded as the climate cooled, leaving the task of drawing the current topography to the glaciers. The combined action of water in liquid form (rivers) and solid form (glaciers) has therefore given the Alpine lakes considerable depths, with the record for Europe held by that of Como with a depth of 410 metres.