Gldhntr- There has been a few archaeologist in the past decade who have claimed they got carbon dates from fire hearths in the 15,000- 20,000 range but none have been accepted as being accurate. The most recent one that I know of was from a cave in the Eastern U.S. (I can't remember the name of the site rite now) They claimed to have some "pre-clovis" period carbon dates but the problem was the cave was on the bottom of a ridge that had extensive mining operations and it's believed that run off from the mining contaminated the site. There has been a number of other claims to... I don't personally think that it's impossible but I am still waiting for proof over the speculation. There are solid "proof" examples of the clovis culture along with carbon dates from all over the country- these also correspond with there being an ice-free corridor leading from Asia to N. Amercia durring the last ice age. There are dozens of these known sites and their artifacts are fairly common (common here is a relative term) but as for a pre-clovs tradition there still is absolutely no hard evidence. For somebody to have artifacts on their land dating from 15,000 - 20,000 years ago there would have had to have been a migration from Asia to N. America durring the previous glaciation (approximately 20,000 B.P.) or possibly a secondary costal migration (but that's a whole nother story!) Out of curriosity I'm wondering why the archaelogist thought you had that old of a site on your land- was it from found artifacts or from carbon dating? If it was from artifacts I would love to see what you have- maybe you could post them? If it turns out to be true we will have to completely re-write the known pre history of N. America and you will have one of the, if not the most important archaeological sites of all time on your land.
Ghost- Deepsix's advice on sifting the potential fire pit (in fact anything you dig in the area) is really good advice. There can be millions of tiny clues in the soil if one just looks for them. Keep a sharp eye out for bone fragments, chips, flakes, seeds, mussel shells, teeth, charcoal, pottery fragments, etc. As far as the remainder of your post.. I don't see the pics- did you post new ones that I'm just not seeing or are you referring to the ones on top? In terms of rocks that don't look like they belong there... I'm trying to picture the area that it's in- (wether or not it was glaciated or not) I believe you are far enough north that this site should be in the glaciated area- if not it would have been on the outwash plain, it either case you can expect to find the unexpected in these areas. The glaciers carried rocks looonnngg distances. The carved or water worn rock that you talk about may very well be a water worn boulder, and indeed look out of place on a hill top in the middle of nowhere but placed there by the glacier. Just as another example- I found a piece of float copper today metal detecting- the closest copper deposits that I know of are quite a ways North from here. Even stranger one could point out that I live in the driftless area (wasn't covered by the last glacier but was part of the outwash plain when it melted) so this little piece of float copper was brought down by the last glacier melted and was deposited on the outwash plain. (The Green Bay lobe of the last glacier actually went farther South than where I live but it was to the East of here. When a glacier advances it does so with finger like extentions, not a straight wall like one might imagine. One lobe of the last glacier stopped to the north of here (just North of the Steven Point area somewhere ((I would have to look on a map to point it out)) and the other lobe ran along the Eastern edge of the county.) All I'm trying to say I guess is that the rocks you see that look out of place may have a natural explanation.
As far as the sitting rock I don't know... I would like to see some closer pictures. It is possible that the "ancient ones" put it there but that would not be my first guess. I have seen rocks pushed up by developing trees- is this a possibility? Jeff said it was definately used- why? (NO, I'm not being a smart a@@ again, I'm serious- is there signs of wear/usage because that could tell a lot about it) Hey- I'm going to get you to start looking at things in a scientific/critical maner vs. speculative if it's the last thing I do!!

I guess I will need some pics and/or more info to make an informed guess on this one.
That mid-state metal detecting competition sure sound like fun... I'm going to have to go to that next year one way or another... you said you found 8 dollars- do they plant silvr dollars too?? That would be cool.. although finding the dimes would be neat too.. is it crowded? I wouldn't want to be metal detecting elbow to elbow with people. How big of an area do they hide the stuff in?
Well, I will be very much looking forward to more posts on what you find.. I will be keeping my fingers crossed, hope you come up with some nice spear points or something.. Need more pictures too!

Good luck!