This is one of those local places that people never seem to go to, but once you do you find it fascinating and wonder why you never went earlier. Their website is full of information if you want to visit:

http://www.stonehengeusa.com/

From their website (I couldn't say it any better):

What is America's Stonehenge?
Built by a Native American Culture or a migrant European population? No one knows for sure. A maze of man-made chambers, walls and ceremonial meeting places, America's Stonehenge is most likely the oldest man-made construction in the United States (over 4000 years old).

Like Stonehenge in England, America's Stonehenge was built by ancient people well versed in astronomy and stone construction. It has been determined that the site is an accurate astronomical calendar. It was, and still can be, used to determine specific solar and lunar events of the year.

Various inscriptions have been found throughout the site including Ogham, Phoenician and Iberian Punic Script. Dr. Barry Fell of Harvard University did extensive work on the inscriptions found at the site. They are detailed in his book America B.C.