I live in Central Wisconsin and have been picking over a local small creek that is just a trove of glacial till. I am rather ignorant still and trying to learn all that I can about geology, but with the winter season approaching I have been really concentrating on just getting out there and making the most of the time I have left, figuring that I'll do most of my self-education over the winter months as I finish cleaning and playing with my finds.
As far as gems and value, I don't know any statistics or variables that would predict what you'd find. I do know that I am absolutely smothered with extremely interesting finds at the moment, most unidentified though I am thinking most of mine are great specimens of colorful petrified wood. There are lots of agate type stones as well as jasper candidates. Quartz seems to abound where I am, as well as calcite. This isn't a definite answer whatsoever, but just wanted to point out that at least for an amateur rock hunter/mineralogy buff, a glacial deposited area can provide quite a lot of experience in identification of a large array of specimens and also satisfaction in at least finding some interesting types. I've been out in other areas that are outside the most obvious till zones, and it's difficult to find anything of obvious interest without a LOT of work and digging, so that is a great option to have. For me, it's a definite joy to have spots within just a 10-minute drive where collecting is legal (and do check out that aspect), the finds are almost guarenteed to interest me if not give me monetary value, and also so productive and varied that I can even take advantage of just an hour of free time.
The facts of variety and also the convenience are the most important for me, since I'm not from this area and other than the glacial deposits, I find it difficult to find anything of interest to me compared to the Kentucky hills and rural areas I grew up in.
So, sorry I didn't answer your question but I'm way too amateur to attempt saying any definite statement about what you COULD find. I just know that in my limited experience, I am extremely happy when picking around my glacial tills, and if nothing else, I'm finding a lot of gorgeous specimens that would be great candidates for lapidary and/or decor--maybe to most true mineralogists they'd be a leaverite, but they're quite likely to be at least the type of leaverite that has a place within sentimental collections or interests. I have only found a few specimens that I'd consider potential GEMS, however. In my glacial till at least, the majority seem to be calcite or pseudomorph types.
Probably a long shot, but I've also heard of glacial diamonds being discovered around here and other glacial areas. I don't know how common they are, and would be more likely to research them just to learn more about the methodology of finding any type of particular specimen in glacial tills, but perhaps there are more common areas for those and other gems.
