186/© Blackwell Science Ltd, GEOLOGY TODAY, September–October 1996 Loess: The Yellow Earth IAN SMALLEY & CHRIS ROGERS A wind-deposited silt forming large deposits in China and middle America, loess is the basis of much grade-one agricultural land. W hat have the following in common? – Buckingham Palace, the collapse of the Teton Dam in Idaho in 1976, the origins of the Chinese civilization, the ‘dustbowl’ of the 1930s in middle America, the economy of New Zealand, and the great 1920 earthquake in Gansu Province in China. The answer is loess, a yellow soil or sediment that is essentially silt- sized (20–60 μm) and deposited by the wind. It used to be said that everybody who had sur- vived a high-school geography course knew three things about loess: it was yellow, depos- ited by the wind and found in China. The facts are still true; the spread of knowledge may be lacking. Loess grows good crops (Iowa is virtually covered in loess) and makes good bricks. Buck- ingham Palace is made of bricks from the loess deposits in North Kent. The locals call it brickearth, but it is true loess. The 90-m-high Teton Dam in Idaho was made of loess, which is not good dam construction material, and the dam failed as the reservoir was being filled. The Chinese civilization, the only one of the ancient civilizations to survive to today, developed in the loess lands of northern China (Fig. 1). Loess suits simple agriculture, and 4000 years ago the loess lands were wetter than now and grew good crops. The loess, having blown into position, can blow away again and this is what happened across the mid-West in the 1930s. Desperately dry conditions and less than per- fect farming practices allowed the surface of some of the valuable land to blow away; luckily much remained, as a major national resource. New Zealand has loess and rain in abundance, and as a result it grows sheep, grapes and trees and survives in a difficult trading world. The great 1920 earthquake in Gansu province mo- bilized the loess ground into huge flowslide movements; the area in motion was about the size of the island of Ireland. Many thousands of people who lived in the easily excavated caves in the loess were killed. In terms of loss of life, it was the worst natural disaster ever to occur. History We need to look at two beginnings of the loess story, an ancient beginning and a compara- tively recent beginning. The ancient Chinese were well aware of the loess and its remarkable properties. They observed the Yellow River with its huge suspended load of loess material (about 40% solids) and noted the vertical fea- tures and cemented nature of the material. The ‘Yellow Earth’ (Hwang tu) was important and, indeed, yellow became the Imperial colour. But there is no record of an ancient land-form sci- ence; the Chinese were astronomers and engi- neers but not, apparently, geomorphologists. The poets took note of the material. The Tang poets of around the eighth century, writing in the Imperial capital at Chang-an, wrote often of dust. The loess dust was everywhere. The dust appears in Chinese poetry as often as water does in English poetry. Chang-an is now Xian and is the site of the Xian Laboratory for Loess and Quaternary Geology (XLLQG), a recent enterprise of the Chinese Academy of Sciences and one of the great centres of loess research. Yellow dust and clear water beneath the Fairy Mountains Change places once in a thousand years which pass like galloping horses. When you peer at far-off China, nine puffs of smoke; Fig. 1. Loess-covered hills near Lanzhou in north-west China (Gansu Province). Even at great heights, cultivation terraces are constructed. (Photo: Tom Dijkstra.)