A gold dredger is a placer mining machine that extracts gold from sand, gravel and dirt by water washing and other mechanical methods.
A large gold dredge uses a mechanical method to dig up material (sand, gravel, dirt, etc.) using steel "buckets" on a circular, continuous, steel "bucket line" at the front of the dredger. The material is then sorted/sifted with water. On large gold dredges, the buckets dump the material into a steel rotating cylinder (a special type of trommel called "the screen") that is sloped downward toward a rubber conveyor belt (the stocker) that carries away oversize material (rocks) and dumps them behind the dredger. The cylinder has many holes drilled into it to allow under-size material (including gold) to fall into a sluice box. The material that is washed away or sorted out is called tailings. The rocks deposited behind the dredger (by the stocker) are called "tailing piles." The holes drilled 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 stocker).
