SAM OGDEN / SCIENCE PHOTO LIBRARY SAM OGDEN / SCIENCE PHOTO LIBRARY
Grain separation. Experiment showing how different grains in a mixture separate into layers, an effect seen during the formation of sedimentary rocks such as sandstone. A mixture of sand & coloured sugar (red) is poured between 2 vertical sheets of glass. The mixture forms a mound whose sides periodically reach a critical angle, causing an avalanche. The smooth grains of sand stop short of the kink at the bottom of the heap, whilst the bigger, rougher sugar grains go a little further. As more grains fall the kink rides upward, forming 2 layers. Each succeeding avalanche makes another pair of layers. This experiment was devised by Hernan Makse of Boston University, USA, in 1997.
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