I found this about 10 inches down, it looks like a feather, or something on one side, I have no idea what kind of metal it is, here is pics of front and back...
My guess is that it's just melted lead, and that the feather image is just coincidental, but I'm only seeing the two-dimensional picture; not the object itself. Interesting. I'll bet that thing about blew your ears off.
I did my research on lake mendota park, and it was a fur trading post in 1832..it was used by Indians can't spell the tribe, before the ho chunk tribe moved in, and they worshiped something called the Thunderbirds...