An 'ancient' Indian arrowhead that I found in my parents' back yard when I was about eight. It's made of black flint. I grew up about 200 yards from an 'ancient' Indian mound, which is about 4,500 years old. There are over 2,600 un-excavated Indian mounds in Ohio, alone. Ohio has the oldest monument in North America: Serpent Mound. It looks like a snake swallowing an egg and the best explanation I've read (guess, really) is that it was built to explain the phases of the moon. The size and scope of the mound is amazing. The Ohio State Parks built a huge metal scaffold with stairs so that one can climb to about 50' in height to view the mound. Nothing was found in it, other than pieces of broken pottery and broken excavation tools that may have been used to build it.
The ancient Indians also built a very large 'fort' of sorts (named: Fort Ancient, which is also an Ohio State Park) not far from here, which had a low wall, completely surrounding their village which had been built to such precision that the height of the remaining wall was less than three inches difference in height at any point. That's simply amazing.