hi bill. i've hit many a spot like that. at first, lots of wheats [usually shallow] and an occasional large cent... just enough to keep me interested. then, BAM! up comes a nice mercury dime or old quarter. then others would follow - and seemingly out of nowhere, and in the most unexpected places. contrary to popular belief, in the early years of our country, people very seldom lost money at all. it was simply too precious to lose. barter, coppers, and spanish silver was king in those days. as the economy grew, so did salaries... and hence, people started losing more pocket change because it was more readily available. it's been estimated that around 15% of all coins minted by the u.s.are in the ground in some way.
it was a big deal for a kid to lose a dime in the 1930's. it easily ruined a saturday with his friends. no bagful of candy today! imagine the impact of a lost dime in the 1830's. that was a lot of money back then! i can easily imagine several people spending a while on their knees combing their yard for that lost dime. most of the coinage we find has been well circulated, and passed through many hands. money had better staying power back then, and inherently was worth more.
simply put, bill, silver is where you find it... and gold is the same way. i know that sounds crass, but it's true. you say you've hunted the park for about 12 hours, which really is not enough time depending on it's size. spread your search out, and push the boundaries. dig all good beeps. it's good you got a different machine. that should help a lot. wait for a good soaking rain, then hit it hard. i hope you are using good quality headphones - they help a lot. good luck in your hunts, and let us know how you do. there's silver in there somewhere. hh!