Complex sentence: Of the five coal seams at Nordegg, two were mined: the No.2 and No.3 Seams, which averaged 7.75 feet (2.4m) and 15.92 feet (4.87m) thick, respectively, and were separated by about 123 feet (37.5m) of rock.
Split into simpler sentences: Of the five coal seams at Nordegg, two were mined: the No.2 and No.3 Seams, which averaged 7.75 feet (2.4m) and 15.92 feet (4.87m) thick, respectively. They were separated by about 123 feet (37.5m) of rock.