The administrative county of Glamorgan existed until April 1 , 1974 . It was replaced with three new counties called Mid Glamorgan , South Glamorgan and West Glamorgan . Under the Local Government Act 1972 , the county boroughs and administrative county of Glamorgan were abolished on 1 April 1974 , with three new counties being established , each containing a former county borough - West Glamorgan , Mid Glamorgan , South Glamorgan .