In lakes, clays and silts generally form the greatest portion of the sediment, with sand occurring near river mouths, along steep slopes and where wave action has sorted out the finer grains. By examining the changing particle-sizes along the length of a core, we get snapshots of changes in the energy of the lake's watershed. Increased sand deposits in cores located a long ways away from the river delta, tell us that either currents and waves increased in intensity and/or frequency or else the lake shore moved closer to the core site, as the lake size shrunk. High clay amounts across the lake basin indicate that little erosion was occurring along the shoreline or in the watershed.
We compare our particle-size results with those from mineral magnetic analyses because both are affected by changes in sediment transport and erosional activity.
Particle-sizes are measured in many ways, from using sieves, to x-rays, to laser beams. Our USGS sediment analysis lab in Denver, Colorado uses a laser instrument called a Malvern Particle Size Analyzer. See the sites below for more information about the technique and the instruments. Check out the Grain Size Results.
U.S. Department of the Interior | U.S. Geological Survey
URL: http://esp.cr.usgs.gov/info/lacs/grainsize.htm
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Page Last Modified: Wed 2-May-2001 10:08:18 MDT