Note the comb-like symbol just below the target. Must be a special kind of dam? I note that is appears at many, though certainly not all, dams. I see it on the 1942 and 1949 USGS topos, after which it just shows up as a straight line at this particular location (the old canal connecting Forge Pond on Nescochague Creek with Pleasant Mills Pond on Hammonton Creek). BTW, There are still remains of the structure at this location, and it shows up rather clearly in the 1940 aerials. It was not present in 1931.