There was a Heavy Rain Today!

I often check in the backyard to see what the rain has washed up. Often bits of glass or ceramic, sometimes a piece of coal. Looked down today and saw something obviously round. Picked it up, washed it off a bit to discover a 1917 silver half-dollar! Now I wonder how long it's been there. It's possible that there is not another living human around today that has touched or seen it...