What is pl. (Mining) The worn sides of river banks, where ores, or stones containing them, accumulate by being washed down from the hills, and thus indicate to the miners the locality of the veins called?

fret — pl. (Mining) The worn sides of river banks, where ores, or stones containing them, accumulate by being washed down from the hills, and thus indicate to the miners the locality of the veins. (Webster's 1913.)