Until recently, most assumed that the American West was a natural dust bowl where every cowboy breathed true grit. Now it seems that the dust was mostly man-made and came with the cows. Head 'em up, move 'em out - and choke on the dust. Before the cows and the cattle trails immortalised in TV series such as Rawhide, there was no dust. It could even explain some of the changes in the region now blamed on global warming.