Hunting an old park produced a pair of 1944 Mercury dimes along with ten wheat pennies.

A close up of the 1944 Mercury dime (top left).  It would grade AU/UNC.  It was probably dropped in 1944 because there is no wear on the coin.  This is another way to more accurately date a site.  If you were to find a well worn 1864 Indian head cent at a park, it would not necessarily mean that the park is that old.  Indian head cents circulated up into the 1940's.  If, on the other hand the 1864 Indian head had very little wear, you can bet that it would date the park to around that time period.