Chesapeake Bay Sediment Distribution - Raster Image

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Chesapeak Bay Sediment Distribution Map


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Explanation of Sediment Distribution Map

    Based on the proportions of sand-, silt- and clay-sized particles, bottom sediments were classified according to Shepard's diagram (see map legend below). Shepard's diagram is an example of a ternary diagram - a device for graphing a three-component system summing to 100%. In this case, the components are the percentages of sand, silt, and clay comprising a sediment sample. Each sediment sample plots as a point within or along the sides of the diagram, depending on its specific grain size composition. A sample consisting entirely of one of the components, 100% sand, for example, falls at the same-named apex. A sediment entirely lacking in one of the components falls along the side of the triangle opposite that apex. The rest fall somewhere in between.

      To classify sediment samples, Shepard (1954) divided a ternary diagram into ten classes. Shepard's diagram follows the conventions of all ternary diagrams. For example, Shepard's "Clays" contain at least 75% clay-sized particles. "Silty Sands" and "Sandy Silts" contain no more than 20% clay-sized particles, and "Sand-Silt-Clays" contain at least 20% of each of the three components. The exact boundaries of each of the ten classes are described in the metadata for the data set used to compile the sediment distribution map.

Map Legend
(Shepard Diagram)

     Shepard, F.P., 1954, Nomenclature based on sand-silt-clay ratios: Journal of Sedimentary Petrology, v. 24, p. 151-158

Examples of how to interpret the Shepard diagram

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