Complex sentence: The largest change to industrial Glamorgan was the opening up of the South Wales coalfield, the largest continuous coalfield in Britain, which occupied the greater part of Glamorgan, mostly north of the Vale.
Split into simpler sentences: The largest change to industrial Glamorgan was the opening up of the South Wales coalfield. The South Wales coalfield is the largest continuous coalfield in Britain, and occupied the greater part of Glamorgan, mostly north of the Vale.