Has your grandpa ever found anything there? Try looking for pottery shards more than the stone type stuff and since the land was part of a treaty and was ceded to settlers you might find colonial type stuff too. I can't tell if what you have so far is Native American. Google images of pottery shards/sherd of the Cherokee and if there is a museum there with Native American artifacts in it go check them out so you can train your eyes. That was a large area and they may not have camped all over it so you might try upstream too. But my advice, since I creek hunt, is that wherever you find modern glass then focus there for old shards and if the stone stuff is there eventually you will find it too, that has been my experience. Good luck!
Stone changes color for a variety of reasons. So the one cracked open? I've seen those before. Coulda been cracked open a while ago- when black flint ages, it gets a white look to it, and lots of sedimentary rock looks like that inside. The others are different types of cobble- like river cobble. Whether or not there's a river nearby, there may have been once. You can find river cobble in all sorts of crazy places you wouldn't expect. Grim knows of what he speaks. And if he's not sure, he'll tell you that too. Keep looking- that's a cool set-up yopu got there by that water- I'd be hanging out there all the time 
