'Black shales' are dark, as a result of being especially rich in unoxidized carbon. Black shales are deposited in anoxic or low oxygen environments, such as in stagnant water columns. They are common in some Palaeozoic and Mesozoic strata, Some black shales contain abundant heavy metals such as molybdenum, uranium, vanadium, and zinc. The heavy metals may have got there by slow accumulation.