Like much of the western world, these areas experienced deindustrialisation during the recession of the 1930s, and then from the Second World War (and particularly from the 1960s) onwards.
Fel llawer o’r byd gorllewinol, aeth y broydd hyn drwy broses o ddad-ddiwydiannu yn ystod Dirwasgiad y 1930au, ac yna o’r Ail Ryfel Byd (ac yn enwedig o’r 1960au) ymlaen.