Cahokia Mounds in southern Illinois. These are actually mounds that are all over southern Illinois but the museum and the mound you can climb on is in Collinsville, IL just over the bridge from St Louis, MO. I only live about 20 minutes south of the main mound.
On back of photo: Sept. 25, 1925 -- "To Uncle John from Edna Earl Gaston.” A later annotation indicates that “Uncle John” was John Clark, a “founder and Senior Warden of St. Mark’s Episcopal Church and an organizer of their parochial school for blacks. Durwood Barbour Collection of North Carolina postcards in the North Carolina Collection, University of North Carolina at Chapel Hill