The Welsh coal owners had failed to invest mechanisation during the good years .
By the 1930s the South Wales Coalfield had the lowest productivity , highest production costs and smallest profits of all Britain 's coal - producing regions .
The Welsh coal owners had failed to invest mechanisation during the good years , and by the 1930s the South Wales Coalfield had the lowest productivity , highest production costs and smallest profits of all Britain 's coal - producing regions .