This image shows the four locks of the Soo Locks complex in Sault Ste. Marie, Michigan

The caption states:
THE SOO LOCKS
Sault Ste. Marie, Michigan

Looking north across the locks to Canada. The first lock is the MacArthur; (named after General Douglas MacArthur) then the Poe; the Davis and the Sabin in that order. The Poe lock is being rebuilt to handle larger freighters which carry iron ore from the rich mines in western Michigan and Minnesota. – Photo by Lucy Gridley

— The notation about the rebuilding of the Poe lock helps to determine the date of the picture, as the lock rebuilding was completed in 1968. This card thus dates from the 1960s.