Over its existence, the administrative county lost territory, both to the existing county boroughs, and also due to the creation of county boroughs at West Hartlepool in 1902 and Darlington in 1915. In 1967 the borough of Hartlepool was removed from the administrative county when it merged with West Hartlepool to form a new county borough of Hartlepool, and in 1968 Billingham was included within the boundaries of the county borough of Teesside, associated with the North Riding.