Well done on the recovery, one just never knows why something like this has been missed.
In a large 400 acre park system in the mid 80's I was told by the dealer/friend that the park was hunted out of all the silvers.
Guys would just dig them and leave the pennies behind.
Though I would still find my share of both.
I could still get silver from that park today given the opportunity.
My 2008 count was 100+ silvers
Query: What type of trees where they?
We used to find large amounts of pennies like this in groupings back in the late 60's early 70's in the school yards.
Explanation from my mother was that in the fall they used to play (1920's) in the piles of maple leaves that had being raked up.