Complex sentence: Virtually identical, they were completed in 1858-1859 on Lake Huron and Georgian Bay and are among the few Lighthouses on the Great Lakes made of cut limestone and granite (not brick, metal, wood or concrete).
Split into simpler sentences: Virtually identical, they were completed in 1858-1859 on Lake Huron and Georgian Bay. The group is among the few on the Great Lakes made of cut limestone and granite (not brick, metal, wood or concrete).