Commercial iron ore exploitation has been relatively uncommon in Wales during the last hundred years, despite the dominance of the iron and steel industry in South Wales. However ironstone is a component of the Lower Coal Measures rock sequence and where it outcrops along the northern edge of the South Wales Coalfield, it was extensively worked for the production of iron and was important in the initiation of the Industrial Revolution in South Wales. Commercial exploitation also took place in the Vale of Glamorgan. The Forest of Dean was an important source of iron for many centuries, and dates from at least the Roman period.