The local concentration of minerals in old river and stream deposits (Alluvium). These form when minerals weathered and transported from the original solid geology deposits are locally concentrated in the bed of steam. These stream deposit may be buried to some considerable depth below subsequent Quaternary gravels. Tin and gold in particular are concentrated in such deposits. Much of the medieval and earlier tin mined in Devon and Cornwall came from stream works working this type of deposit.
Related terms: Eluvial, Colluvial