Complex sentence: One of the earliest sites of historical interest in the valley is that of the dykes at Broadclough, which are associated with the Battle of Brunanburh, later the valley was part of the medieval Forest of Rossendale.
Split into simpler sentences: One of the earliest sites of historical interest in the valley is that of the dykes at Broadclough, which are associated with the Battle of Brunanburh. Later, the valley was part of the medieval Forest of Rossendale.