Complex sentence: By that time, the mill at Birdsedge was a major employer in the area, with the majority of stone cottages (erected in the 19th century) housing workers who had moved from outlying areas to find employment.
Split into simpler sentences: By that time, the mill at Birdsedge was a major employer in the area. The majority of 19th century stone cottages housed workers who had moved from outlying areas to find employment.