How We Found Out There Were Ice Ages

Step 1 of 5

The boulders made no sense.

All across northern Europe there were huge stray rocks made of stone found nowhere nearby. They were too big for a river and too far from any mountain. The word for them, erratics, simply means wanderers.

Step 2 of 5

The first explanation was a flood — and it did not fit.

A great flood was the obvious answer, and it was widely accepted. But water drops heavy things first and light things last, and these boulders were sitting on top of fine sediment, not underneath it. Water sorts. Whatever moved these had not sorted anything.

Step 3 of 5

Then someone recognised the scratches.

Louis Agassiz had grown up near Swiss glaciers and knew what glacier-scraped rock looks like: parallel grooves, all running the same way, gouged by stones frozen into the ice. The same grooves were on bedrock all over Europe, far from any glacier.

Step 4 of 5

Living glaciers were doing all of it, in miniature.

Every feature was already visible in the Alps: boulders carried and dumped, ridges of unsorted rubbish pushed up at the snout, U-shaped valleys. The argument was not a new theory — it was pointing at a working example and saying: that, but everywhere.

Step 5 of 5

Nobody believed it for twenty years.

The idea that ice had covered half a continent was thought absurd, and Agassiz was attacked for it. The evidence kept accumulating — and the deciding proof came much later from ice cores and ocean sediments, showing dozens of ice ages, not one.

We are in an ice age now, by the geological definition — there is permanent ice at both poles, which for most of Earth's history there was not.