Glacial erosion dramatically alters mountain landscapes, but the pace at which glaciers carve a previously fluvial landscape remains poorly defined because long-term valley incision rates are difficult to measure. Here we reconstruct the lowering history of the Aare Valley, Switzerland, over the past 4 m.y. by dating cave sediments with cosmogenic 26Al and 10Be. Incision accelerated from ∼120 m/m.y. to ∼1200 m/m.y. at 0.8–1.0 Ma, at least 1 m.y. after the onset of local glaciation. Rapid incision may have been triggered by lowering of the equilibrium line altitude at the mid-Pleistocene climate transition.