Simplify the following text for a Grade 11 native English-speaking person.
Original: The Vale of the White Horse district and parts of the South Oxfordshire administrative district south of the River Thames were historically part of Berkshire , but were added to the administrative county of Oxfordshire in 1974 .
Simplified: The Vale of the White Horse and parts of South Oxfordshire south of the River Thames were historically part of Berkshire , but were added to the administrative county in 1974 .