From National Geographic:
Among their latest finds: Like Stonehenge, the Ohio timber circles were likely used to mark astronomical events such as the summer solstice.
Formally called Moorehead Circle but nicknamed “Woodhenge” by non-archaeologists, the site was once a leafless forest of wooden posts. Laid out in a peculiar pattern of concentric, but incomplete, rings, the site is about 200 feet (57 meters) wide. (See a picture of reconstructed timber circles near Stonehenge.)
Today only rock-filled postholes remain, surrounded by the enigmatic earthworks of Fort Ancient State Memorial (map). Some are thousands of feet long and all were built by Indians of the pre-agricultural Hopewell culture, the dominant culture in midwestern and eastern North America from about A.D. 1 to 900.
This year archaeologists began using computer models to analyze Moorehead Circle's layout and found that Ohio's Woodhenge may have even more in common with the United Kingdom's Stonehenge than thought—specifically, an apparently intentional astronomical alignment.