Six million years ago, massive erosion of the Alps mountains occurred by a sudden drop in the level of the Mediterranean Sea, a team led by Sean Willett, a University of Washington geologist discovered.
With erosion more or less keeping pace with the tectonic forces that enlarge the mountains, the mountain ranges typically reach a sort of equilibrium.
Here is an excerpt explaining how it all happened:
But an event called the Messinian salinity crisis, precipitated by blockage of the forerunner of the Strait of Gibraltar, cut the Mediterranean off from the rest of the world’s oceans. Evaporation greatly reduced the water level, dropping it as much as two or three miles below the rest of the world’s ocean surfaces.
The beds of rivers flowing from the Alps dropped sharply as the level of the Mediterranean basin fell, and their force carried away huge amounts of sediment. The forces carved many of the distinctive deep valleys for which the Alps are known and left behind nearly a dozen major alpine lakes in the ssouthern Alps.