Complex sentence: The location of the county hall has usually denoted the county town, and as county halls have moved it has also been considered that the county town has moved, for example when Derbyshire County Council moved from Derby to Matlock in the 1950s.
Split into simpler sentences: The location of the county hall has usually denoted the county town. When county halls have moved, this has been taken as a sign that the county town has moved, as when Derbyshire County Council moved from Derby to Matlock in the 1950s.