Frosted sand grains quietly reveal that the wind once carried them
Look at a quartz grain under a scanning electron microscope and its surface tells a travel story. Polished grains and frosted ones have had different journeys, and a frosted finish is a classic sign of windblown sand. Every particle of sediment carries clues to how it moved and where it settled.
Sediment is loose solid material broken off by weathering and erosion, then moved by water, wind, ice or plain gravity until it comes to rest. Rivers do most of the carrying, building beaches and channel deposits or dropping particles in still lakes and seas. Wind builds dune fields and blankets of loess, and glaciers bulldoze everything from fine grit to boulders into moraines. Buried deep enough, sand and silt harden into sandstone and siltstone.
Geologists describe grains by size, shape and makeup. Size runs on a base-2 logarithmic scale called Phi, from colloids up to boulders. Shape has three parts: overall form, such as spherical, platy or rodlike; roundness, meaning how sharp the corners are, usually judged against comparison charts because the exact formulas are awkward; and surface texture, the pits, ridges and scratches that quartz preserves especially well. William Krumbein devised a single number for form that equals 1 for a perfect sphere.
Whether a grain travels or drops depends on the flow's strength and the particle's size, density and shape. The Exner equation links the rise of a riverbed to how much sediment falls out of the current, which explains scour holes behind boulders and deposits on the inside of meander bends. A dam makes the river pool and dump its whole load. In the open ocean, far from rivers, the shells of dead organisms supply most of the sediment, and lake muds that never became rock can reveal past climates.
Human land use can turn erosion into a crisis. On Madagascar's high central plateau, roughly a tenth of the country, stripped land has split into gullies called lavakas, typically about 40 metres wide, 80 long and 15 deep, sometimes up to 150 per square kilometre. They may supply 84 percent of the sediment in local rivers, turning them red-brown and killing fish. Clearing away eroded sediment in the EU and UK probably costs more than 2.3 billion euros a year.
Source: Sediment
