X-ray radiographs of box cores of shelf sand off the coast of California and Oregon show linear concentrations of grains opaque to X-rays. A number of the lines cross one another, suggesting that they are of biogenic origin. Experimental studies show that infaunal polychaetes such as Nephtys produce similar concentrations of heavy minerals as they move through sand. A similar origin is postulated for the observed concentrations in the shelf-sand box cores.

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