Material removed or excavated by dredging; the debris, silt, or other matter brought up from the bottom of a water body during dredging operations.
Gerund/present participle of 'dredge' converted to a noun, referring to the actual material or process. The '-ing' form can be nominalized in English to refer to the stuff produced by an action.
Dredgings from harbors often contain interesting archaeological artifacts and also serious pollution—they're literally a record of what humans have thrown into the water for centuries.
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