Loess is more

Glaciers can have a powerful effect across thousands of kilometres as well as nearby. Take loess. It is pronounced low-ess, and in small volumes looks suspiciously like dust. In fact its grains are somewhere in size between sand and clay. It was formed in vast amounts by the grinding action of ice during the last, and other, ice ages.

But once formed, loess would not stay still. Instead, its grains were small enough to be blown by the wind, often for thousands of kilometres, until it formed deep deposits in the areas where the wind dropped its load. Loess deposits now cover significant areas of China, eastern Europe and the US. About 30 percent of the contiguous US has at least one loess layer and often more, and the different layers have been dated to reveal which glaciation episode caused them.

These deposits can be tens of metres deep. The areas where they occur are sought-after because loess is rich in nutrients and forms fertile soils. But there is a down side. Because the material is small-grained and contains lots of shiny quartz crystals it has very little internal friction. In China, the Loess Plateau has long been a productive agricultural region, but it has been subject to severe erosion because of the material's poor stability. It is surrounded on three sides by the Hwang He River, whose English name, the Yellow River, comes from the vast amounts of loess it carries away. When laid down in thick deposits, it can form unstable steep structures. In China in 1920, an earthquake caused a loess landslide that is blamed for killing 100,000 people, and similar incidents with smaller death tolls continue.