Although it’s pretty obvious that it’s the scouring action of the rivers of ice that forms glacial valleys, the precise details of how this happens over time have long eluded geologists. In the present study, which was published in Science, the scientists used a new dating technique to reconstruct the landform history of a 750-square-kilometre area of Fiordland in New Zealand, from the early Pleistocene, some 2.5 million years ago, when the world cooled and glaciers formed, through to the present day.

The technique, known as helium-4/helium-3 thermochronometry, enabled the scientists to determine the temperatures of rocks as they cooled over geological time. Temperature increases with depth, so this thermal history gives an indication of how deeply the rocks were buried over a period of millions of years.

The range of temperatures exhibited by the rock samples they collected were tested against various scenarios for the formation of Fiordland’s landscape. The only one that fitted suggested that over the first million years, most of the erosion took place at the mouths of the glaciers, as they scoured the U-shaped valleys on the flanks of the mountain range. Then, for the next million years or so, erosion took place primarily at the heads of glaciers as they ate into their headwalls, creating the steep, amphitheatre-like cirques.

May 11