Zinc cent decay question

I was traveling on Monday and stopped at a large public park in another state. On the near 200 acres, there are several old buildings. I spent less than an hour near a building built in the mid-1600's. It was only a few feet from parking lots and paved paths. The park is heavily used (baseball fields, walking paths, playgrounds, picnic areas, etc). Figured it had already been searched, but I tried anyway. I assumed I would find nothing or modern coins on the surface. Got signals everywhere and dug up 15 coins. They ranged from 3 to 5 inches deep. Nothing older than 1950.

Sorry for the poor quality of the cell phone pic. It shows some Lincoln Memorial cents that are corroded/decayed. Some are missing edges; others holed right through the coin.

How long does it take for these things to decay this much? They were only a few inches down.