Dry-Land Gold Dredge
During the thirties equipment called dry-land dredge was used in California to recover gold. Most of them were poorly designed and constructed that they had no chance to succeed and were used for very short periods. Gravel accumulated under the skids, irregular bedrock interfered and much time was lost in moving. Another common problem was tailing sluices on trestles, which needed rebuilding every time a move was made. Lack of head room often resulted in the tailing backing up against the rear end of the washing plant. Most of these outfits were built of second hand material including second hand gasoline engines.
The last models were operated with some degree of success such as the one used in the Lincoln district, Placer County. The outfit included a movable land plant, not self propelled, consisting of a hopper, trommel, and stackers. It was mounted on a steel frame supported at the rear on caterpillar treads. The trommel consisted of two screens, one inside the other. The inner trommel was perforated with 1-inch holes and the outer screen was perforated with 1 1/8 inch slots. The undersize was sent to the gold recovery circuit.
These plants were designed to operate on dry ground and were not standardized. Another somewhat similar method of handling placer gravel was hauling it with trucks to a stationary washing plant consisting of a hopper, trommel and riffle sluices for recovering gold. The oversize from the trommel was usually discharged into a bin from which it was hauled away with trucks. This method was used when gravel was to tight to dig with a dragline or if other conditions were unfavorable to the use of a dragline, but the cots was so high that the gold content of the gravel should be higher than that needed for dragline dredging.