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A gold dredge is a placer mining machine that extracts gold from sand, gravel, and dirt using water and mechanical methods.
The original gold dredges were large, multi-story machines built in the first half of the 1900s.
Small suction machines are currently marketed as "gold dredges" to individuals seeking gold: just offshore from the beach of Nome, Alaska, for instance.
A large gold dredge uses a mechanical method to excavate material (sand, gravel, dirt, etc.) using steel "buckets" on a circular, continuous "bucketline" at the front end of the dredge. The material is then sorted/sifted using water. On large gold dredges, the buckets dump the material into a steel rotating cylinder (a specific type of trommel called "the screen") that is sloped downward toward a rubber belt (the stacker) that carries away oversize material (rocks) and dumps the rocks behind the dredge. The cylinder has many holes in it to allow undersized material (including gold) to fall into a sluice box. The material that is washed or sorted away is called tailings. The rocks deposited behind the dredge (by the stacker) are called "tailing piles." The holes in the screen were intended to screen out rocks (e.g., 3/4 inch holes in the screen sent anything larger than 3/4 inch to the stacker).